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/testmultiaudio.c

@@ -199,3 +199,5 @@ main(int argc, char **argv)
     SDL_Quit();
     return 0;
 }
+
+/* vi: set ts=4 sw=4 expandtab: */