OS: Fix used resource debug prints

These methods were broken by 22419082d9
5 years ago and nobody complained, so maybe they're not so useful...
But at least this should restore them to a working state.

core/io/resource.cpp

@@ -552,5 +552,7 @@ void ResourceCache::dump(const char *p_file, bool p_short) {
 	}
 
 	lock.read_unlock();
+#else
+	WARN_PRINT("ResourceCache::dump only with in debug builds.");
 #endif
 }