include: Make function pointer params into typedefs.

This is easier to read and document in general, but will also make some
new parsing work in wikiheaders much easier.

src/thread/SDL_thread.c

@@ -43,7 +43,7 @@ void *SDL_GetTLS(SDL_TLSID id)
     return storage->array[id - 1].data;
 }
 
-int SDL_SetTLS(SDL_TLSID id, const void *value, void(SDLCALL *destructor)(void *))
+int SDL_SetTLS(SDL_TLSID id, const void *value, SDL_TLSDestructorCallback destructor)
 {
     SDL_TLSData *storage;