Fix warning messages

[btd1337]

Please, consider fix this warnings when meson build --prefix=/usr is used

Found pkg-config: /usr/bin/pkg-config (1.5.2)
Native dependency gstreamer-1.0 found: YES 1.14.2
Native dependency gstreamer-base-1.0 found: YES 1.14.2
Native dependency gstreamer-audio-1.0 found: YES 1.14.2
Native dependency gstreamer-fft-1.0 found: YES 1.14.2
Native dependency glib-2.0 found: YES 2.56.1
Native dependency gobject-2.0 found: YES 2.56.1
Native dependency gmodule-2.0 found: YES 2.56.1
Native dependency gee-0.8 found: YES 0.20.1
Dependency glib-2.0 found: YES (cached)
Dependency gobject-2.0 found: YES (cached)
Native dependency gio-2.0 found: YES 2.56.1
Dependency gee-0.8 found: YES (cached)
Library posix found: YES
Native dependency libpulse found: YES 12.2
Native dependency libpulse-mainloop-glib found: YES 12.2
Dependency gstreamer-1.0 found: YES (cached)
Dependency gstreamer-audio-1.0 found: YES (cached)
Native dependency wingpanel-2.0 found: YES 2.0
Native dependency switchboard-2.0 found: YES 2.3.1
WARNING: Passed invalid keyword argument "default".
WARNING: This will become a hard error in the future.
WARNING: Passed invalid keyword argument "default".
WARNING: This will become a hard error in the future.
Native dependency granite found: YES 5.0
Native dependency gtk+-3.0 found: YES 3.22.30
Native dependency libcanberra found: YES 0.30
Native dependency libcanberra-gtk found: YES 0.30
Native dependency libwnck-3.0 found: YES 3.24.1
Dependency granite found: YES (cached)
Dependency libwnck-3.0 found: YES (cached)
Dependency granite found: YES (cached)
Dependency libwnck-3.0 found: YES (cached)
Library posix found: YES
Dependency granite found: YES (cached)
Dependency gtk+-3.0 found: YES (cached)
Dependency granite found: YES (cached)
Dependency gtk+-3.0 found: YES (cached)
Dependency gstreamer-1.0 found: YES (cached)
Dependency granite found: YES (cached)
Dependency gtk+-3.0 found: YES (cached)
Dependency gstreamer-1.0 found: YES (cached)
Build targets in project: 15
Found ninja-1.8.2 at /usr/bin/ninja

and ninja

[29/95] Compiling Vala source ../lib/PulseAudio/Operations.vala ../lib/PulseAudio/M...alizer.vala ../lib/PulseAudio/Spectrum.vala lib/com.gitlab.mithrandirn.hottoe.vapi.
../lib/PulseAudio/Channel.vala:89.5-89.46: warning: Type `PulseAudio.CVolume?' can not be used for a GLib.Object property
    public global::PulseAudio.CVolume? cvolume { get; set; default = null; }
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
../lib/PulseAudio/Channel.vala:90.5-90.53: warning: Type `PulseAudio.ChannelMap?' can not be used for a GLib.Object property
    public global::PulseAudio.ChannelMap? channel_map { get; set; default = null; }
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
../lib/PulseAudio/Plug.vala:69.5-69.46: warning: Type `PulseAudio.CVolume?' can not be used for a GLib.Object property
    public global::PulseAudio.CVolume? cvolume { get; set; default = null; }
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
../lib/PulseAudio/Plug.vala:70.5-70.53: warning: Type `PulseAudio.ChannelMap?' can not be used for a GLib.Object property
    public global::PulseAudio.ChannelMap? channel_map { get; set; default = null; }
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Compilation succeeded - 4 warning(s)