Remove SDL_config.h from the public headers

The SDL headers are no longer dependent on the build configuration.

Fixes https://github.com/libsdl-org/SDL/issues/6643 and https://github.com/libsdl-org/SDL/issues/6641

test/testkeys.c

@@ -12,10 +12,7 @@
 
 /* Print out all the scancodes we have, just to verify them */
 
-#include <stdio.h>
-#include <ctype.h>
 #include <stdlib.h>
-#include <string.h>
 
 #include "SDL.h"
 
@@ -38,3 +35,5 @@ main(int argc, char *argv[])
     SDL_Quit();
     return (0);
 }
+
+/* vi: set ts=4 sw=4 expandtab: */