Fixed using files object in subdir with xcode backend
Fixed using files object in subdir with xcode backend

Fixes bug #589. When generating string from file object, it didn't take subdir
into account.

pexip/meson,pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,QuLogic/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,QuLogic/meson,QuLogic/meson,mesonbuild/meson,jpakkane/meson,mesonbuild/meson,jpakkane/meson,jpakkane/meson,QuLogic/meson,jpakkane/meson,jpakkane/meson,pexip/meson,jpakkane/meson,pexip/meson,QuLogic/meson,jpakkane/meson,QuLogic/meson,mesonbuild/meson,mesonbuild/meson,jpakkane/meson,QuLogic/meson,pexip/meson,pexip/meson,mesonbuild/meson,QuLogic/meson,QuLogic/meson,jpakkane/meson