Fix compilation error on old SDL2 library.

src/modules/system/sdl/System.cpp

@@ -25,9 +25,12 @@
 // SDL
 #include <SDL_clipboard.h>
 #include <SDL_cpuinfo.h>
-#include <SDL_locale.h>
 #include <SDL_version.h>
 
+#if SDL_VERSION_ATLEAST(2, 0, 14)
+#include <SDL_locale.h>
+#endif
+
 namespace love
 {
 namespace system