g-i fixes

Make g-i less spammy (in meson mostly):

commit 2d24674b:

g-i: pass --quiet to g-ir-scanner
This suppresses the annoying 'g-ir-scanner: link: cc ..' output
that we get even if everything works just fine.

We still get g-ir-scanner warnings and compiler warnings if
we pass this option.

commit d99994a8:

validate: fix g-i warnings
gst-validate-utils.c:914: gst_validate_element_matches_target: unknown parameter 'structure' in documentation comment, should be 's'
gst-validate-override-registry.h:49: gst_validate_override_registry_get_override_list: return value: Missing (element-type) annotation

commit 9521fdbf:

meson: validate: actually pass extra arguments to gnome.generate_gir()
Especially the init section and the --quiet.

Remove the whole manual build/source dir include addition
to the g-ir-scanner args seeing that things worked fine
without the args being passed to the scanner at all.