anv: Prefer in-tree headers to out-of-tree headers

anv: Prefer in-tree headers to out-of-tree headers

Set the include paths to consider in-tree headers before out-of-tree
headers.

Avoids the build failing due to stale headers being present in
$prefix. Previosuly 'make -ki install' or something similar was required
to update the out-of-tree headers to allow the build to succeed.

Also avoids having to rebuild the entire thing after every 'make
install'.

Cc: Rob Clark <robdclark@gmail.com>
Cc: Jason Ekstrand <jason.ekstrand@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Chad Versace <chadversary@chromium.org>