Fix implicit conversion changes signedness: 'gboolean' to 'guint'

[rbuj]

CFLAGS="-Wconversion -Wunused-macros -Wunused-parameter" CC=clang ./autogen.sh --prefix=/usr --enable-debug --enable-compile-warnings=maximum && make &> make.log

gsd-media-keys-window.c:178:46: warning: implicit conversion changes signedness: 'gboolean' (aka 'int') to 'guint' (aka 'unsigned int') [-Wsign-conversion]
                window->priv->volume_muted = muted;
                                           ~ ^~~~~
--
msd-osd-window.c:444:39: warning: implicit conversion changes signedness: 'gboolean' (aka 'int') to 'guint' (aka 'unsigned int') [-Wsign-conversion]
        window->priv->is_composited = gdk_screen_is_composited (screen);
                                    ~ ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[lukefromdc]

My only laptop is a converted Chromebook (chromeOS totally replaced w Debian Unstable) so the media and brightness keys do not work. Thus I cannot test the brightness OSD but w two other approving reviews will merge this. Ideally someone with a laptop and working media keys would test this with the brightness applet but I don't see that as likely to happen at this point, and I cannot do it myself