Skip g_subprocess_launcher_set_child_setup() in introspection

It's not likely that the runtime of a bound language using the
introspection supports running in a process forked by a foreign
library, so that a closure programmed in that language would work
safely.

Any programming environment supporting that would probably have
its own advanced facilities for process spawning, or be able
to access the GLib spawning APIs via raw C bindings (still
represented in the introspection, (skip) only adds a flag)
and do any low-level preparatory dances as necessary for the
forked runtime.

Note that there are other APIs making use of GSpawnChildSetupFunc,
but they are usable with the closure nullified, and we cannot annotate
the closure parameters away because that would break the annotated API
for bindings; accordingly to bug #738176 comment #3, the current bindings'
users are expected to pass null.