build: Fix path handling in pkgconfig

On Nix, headers, introspection files and vapi files are installed to different
prefix than the library. This makes sure that the correct paths are used, thus
making the build more portable.
1 job for wip/jtojnar/meson-fixes in 6 minutes and 10 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed #209445
build_stable

00:06:10