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.