meson: Use GirTarget for generate_gir() includes.
Meson currently can't locate the uninstalled .gir files resulting in the following type of errors:
Package gstreamer-1.0 was not found in the pkg-config search path. Perhaps you should add the directory containing gstreamer-1.0.pc' to the PKG_CONFIG_PATH environment variable No package 'gstreamer-1.0' found
Lookup the GirTarget from the subproject and use that instead.