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

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);
                                    ~ ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
--
msd-timeline.c:649:16: warning: implicit conversion changes signedness: 'gboolean' (aka 'int') to 'guint' (aka 'unsigned int') [-Wsign-conversion]
  priv->loop = loop;
             ~ ^~~~

[raveit65]

I can confirm that mentioned warnings are gone.