meson.build: fix building as a subproject
When building as a subproject:
meson.build_root()
-> /home/tdeo/sway/build
meson.source_root()
-> /home/tdeo/sway
meson.current_source_dir()
-> /home/tdeo/sway/subprojects/libinput
meson.current_build_dir()
-> /home/tdeo/sway/build/subprojects/libinput