Update xxhash.cc to allow combined compilation (#5969)

Summary:
To fix unity_test
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5969

Test Plan: make unity_test

Differential Revision: D18140426

Pulled By: pdillinger

fbshipit-source-id: d5516e6d665f57e3706b9f9b965b0c458e58ccef

util/xxhash.cc

@@ -119,7 +119,10 @@ static void* XXH_memcpy(void* dest, const void* src, size_t size) { return memcp
 
 #include <limits.h>   /* ULLONG_MAX */
 
+#ifndef XXH_STATIC_LINKING_ONLY
 #define XXH_STATIC_LINKING_ONLY
+#endif
+
 #include "xxhash.h"
 
 /* BEGIN RocksDB customizations */