buddy_test: output OK to stdout

such that the test can easily be run with 2>/dev/null if the dbg output
is of no interest.

src/buddy.c

@@ -2814,6 +2814,6 @@ int main(void)
 	t_buddy((1 << 15) + 128);
 	t_buddy((1 << 15) + 196);
 	t_buddy((1 << 17) + 9763);
-	fprintf(stderr, "test OK\n");
+	printf("OK\n");
 }
 #endif