build: Use newer glib-genmarshal argument
build: Use newer glib-genmarshal argument

When using glib-genmarshal from GLib 2.54, we can use the newly added
`--prototypes` command line argument to generate the prototypes of the
marshallers in the C source, and void a missing-prototypes compiler
warning.

GNOME/clutter,GNOME/clutter,GNOME/clutter,GNOME/clutter,GNOME/clutter