girparser: support return_void metadata argument
girparser: support return_void metadata argument

This allows us to generate backwards compatible signatures for
functions with a void return value and a single out param for a struct
from metadata files instead of resorting to *-custom.vala.

GNOME/vala,frida/vala,frida/vala,GNOME/vala,frida/vala,spidermonkdust/vala,frida/vala,rvu95/vala,GNOME/vala,flobrosch/vala-fixes,spidermonkdust/vala,spidermonkdust/vala,GNOME/vala,flobrosch/vala-fixes,frida/vala,rvu95/vala,flobrosch/vala-fixes,rvu95/vala