gallium: copy xxhash.h from Mesa

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Ryan Neph <ryanneph@google.com>
Reviewed-by: Gert Wollny <gert.wollny@collabora.com>

src/gallium/meson.build

@@ -69,6 +69,7 @@ sources_libgallium = [
    'auxiliary/util/u_prim.h',
    'auxiliary/util/u_debug_describe.c',
    'auxiliary/util/u_atomic.h',
+   'auxiliary/util/xxhash.h',
    'auxiliary/cso_cache/cso_hash.h',
    'auxiliary/cso_cache/cso_cache.h',
    'auxiliary/cso_cache/cso_cache.c',