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.
32 jobs for !246 with cleanup_gir in 31 seconds (queued for 2 seconds)
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