BUILD: stick-tables: fix build breakage in xxhash on older compilers

BUILD: stick-tables: fix build breakage in xxhash on older compilers

Commit 36d156564 ("MINOR: peers: Support for peer shards") reintroduced
a direct dependency on import/xxhash.h which was previously dropped by
commit d5fc8fcb8 ("CLEANUP: Add haproxy/xxhash.h to avoid modifying
import/xxhash.h"). This results in xxhash.h being included twice, which
breaks the build on older compilers which do not like seeing XXH32_hash_t
being defined twice.

Let's just use include/haproxy/xxhash.h instead.

No backport is needed.