anv: automake: bring back VULKAN_ENTRYPOINT_CPPFLAGS

anv: automake: bring back VULKAN_ENTRYPOINT_CPPFLAGS

We should not have removed them in the first place. There's a subtle
difference between generating the complete sources and using them which
was not obvious as we nuked them.

Without this, the release tarball ends up without various hunks of the
generated sources, thus things fail at a later stage as we attempt to
build them.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>