gobject-introspection: use same PKG_CONFIG_PATH for g-ir-compiler as for g-ir-scanner

gobject-introspection: use same PKG_CONFIG_PATH for g-ir-compiler as for g-ir-scanner

Make sure to use the PKG_CONFIG_PATH set at configure time instead of
just relying on an env-var set one. This makes sure both g-ir-compiler
and g-ir-scanner use the same PKG_CONFIG_PATH for determining include
paths etc.