BLocker: make "unlock from wrong thread" only a warning.

Calling Debugger here causes issues with some software (mainly Mesa).
Only print a warning, this is enough for developers to see the problem,
and not as annoying for users.

src/kits/support/Locker.cpp

@@ -112,9 +112,11 @@ void
 BLocker::Unlock()
 {
 	// The Be Book explicitly allows any thread, not just the lock owner, to
-	// unlock. This is bad practice and Haiku should not allow it.
+	// unlock. This is bad practice, but we must allow it for compatibility
+	// reasons. We can at least warn the developer that something is probably
+	// wrong.
 	if (!IsLocked())
-		debugger("Trying to unlock from the wrong thread (#6400)");
+		fprintf(stderr, "Trying to unlock from the wrong thread (#6400)");
 
 	// Decrement the number of outstanding locks this thread holds
 	// on this BLocker.