tests: Fix out-of-tree builds
tests: Fix out-of-tree builds

Make sure we include any generated headers from src/ as well, like
wayland-version.h.

Signed-off-by: Daniel Stone <3d0f3b9ddcacec30c4008c5e030e6c13a478cb4f@fooishbar.org>

tulcod/wayland,tulcod/wayland,sir-murray/wayland,eyolfson/wayland,mchalupa/wayland,bitfactor/wayland,tulcod/wayland,mchalupa/wayland,eyolfson/wayland,abooij/wayland,sir-murray/wayland,eyolfson/wayland,abooij/wayland,bitfactor/wayland,sir-murray/wayland,mchalupa/wayland,abooij/wayland,bitfactor/wayland