Fix linking due to clashing xxhash defines between LZ and Zstd.

Probably would be better to use XXH_PRIVATE_API instead, but that requires xxhash.c to have that exact name without prefixes.

C/lz4/lz4xxhash.c

@@ -105,6 +105,7 @@ static void  XXH_free  (void* p)  { free(p); }
 static void* XXH_memcpy(void* dest, const void* src, size_t size) { return memcpy(dest,src,size); }
 
 #define XXH_STATIC_LINKING_ONLY
+#define XXH_NAMESPACE lz
 #include "xxhash.h"