meson.build: fix gir install dir
This has always been broken, apparently, in the meson build.
Refer to the discussion here https://github.com/mesonbuild/meson/pull/9484#issuecomment-980131791
Went unnoticed until some internal changes in meson 0.60.x branch exposed the issue (build currently fails at the start because of this)
Edited by Paul Mulders