Avoid compiler warning / fix build when building without xxhash

Fixes #48

src/fellow_log_test.c

@@ -781,10 +781,12 @@ main(int argc, char **argv)
 	double t;
 	const double tmax = 1;
 	void *priv = NULL;
+#ifdef HAVE_XXHASH_H
 	uint8_t fht[2];
 
 	fht[0] = fh_name[FH_XXH3_128] ? FH_XXH3_128 : FH_XXH32;
 	fht[1] = fh_name[FH_XXH3_64]  ? FH_XXH3_64  : FH_XXH32;
+#endif
 
 	if (argc == 2 && *argv[1] != '-')
 		filename = argv[1];