rts: define XXH_INLINE_ALL

This commit cleans up how we include the xxhash.h header and only
define XXH_INLINE_ALL, which is sufficient to inline the xxHash
functions without symbol collision.

(cherry picked from commit ee01de7d)

rts/Hash.c

@@ -26,9 +26,7 @@
 #endif
 #endif
 
-#define XXH_NAMESPACE __rts_
-#define XXH_STATIC_LINKING_ONLY   /* access advanced declarations */
-#define XXH_IMPLEMENTATION   /* access definitions */
+#define XXH_INLINE_ALL
 
 #include "xxhash.h"