When building against a glib-genmarshal from GLib 2.54 we can use the
`--prototypes` command line argument to generate the prototypes for the
marshallers in the C source, and avoid a missing-prototypes compiler
warning.
GLib now generates the prototypes for the generated marshallers, so it's
not necessary to include the header any more.
This fixes a build failure in GNOME Continuous with GLib master, caused
by -Werror=redundant-decls.