Fixed bug 3010 - SDL_x11messagebox.c needs including X11/keysym.h

Ozkan Sezer

SDL_x11messagebox.c needs including <X11/keysym.h> otherwise XK_Escape,
etc might not be available to it. Seen this with x.org-x11-6.8.2.

src/video/x11/SDL_x11messagebox.c

@@ -28,6 +28,7 @@
 #include "SDL_x11dyn.h"
 #include "SDL_assert.h"
 
+#include <X11/keysym.h>
 #include <locale.h>