glib-genmarshal/glib-mkenums: Add comment clarifying licensing

Clarify the licensing of the code generated by the two scripts in a
comment in the header of each generated file. The intention is that the
license of GLib does *not* apply to the generated files; but that they
are subject to the linking restrictions of the LGPL, since they link to
GLib and GLib is licensed under the LGPL. The generated files themselves
are under the license of whatever project they’re generated for.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=788990