Update _XData32 declaration in SDL_x11sym.h to match that of libx11 1.6.

--- SDL-1.2.15/src/video/x11/SDL_x11sym.h.~1~	2012-01-19 01:30:06.000000000 -0500
+++ SDL-1.2.15/src/video/x11/SDL_x11sym.h	2014-12-26 00:22:36.445067694 -0500
@@ -165,7 +165,7 @@
  */
 #ifdef LONG64
 SDL_X11_MODULE(IO_32BIT)
-SDL_X11_SYM(int,_XData32,(Display *dpy,register long *data,unsigned len),(dpy,data,len),return)
+SDL_X11_SYM(int,_XData32,(Display *dpy,register _Xconst long *data,unsigned len),(dpy,data,len),return)
 SDL_X11_SYM(void,_XRead32,(Display *dpy,register long *data,long len),(dpy,data,len),)
 #endif
 
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