Only convert the result of XLookupString() if it's not already UTF-8

Fixes https://github.com/libsdl-org/SDL/issues/7766

src/SDL_utils_c.h

@@ -26,6 +26,9 @@
 /* Common utility functions that aren't in the public API */
 
 /* Return the smallest power of 2 greater than or equal to 'x' */
-int SDL_powerof2(int x);
+extern int SDL_powerof2(int x);
+
+/* Return whether the string is valid UTF8 */
+extern SDL_bool SDL_utf8valid(const char *str, size_t bytes);
 
 #endif /* SDL_utils_h_ */