In the DEPS file we are going to use Chromium's mirror of
github.com/KhronosGroup/SPIRV-Cross so we need to adjust .gitmodules to
point to it instead of Kangz/SPIRV-Cross. Also take that opportunity to
roll SPIRV-Cross.
Change-Id: I4f53a4fc54f5b1b9a754ac55e976c81e5eeabeb2