stdinc: modify default alloca prototype

For compatibilty with TinyCC.

include/SDL3/SDL_stdinc.h

@@ -66,7 +66,7 @@
 #  elif defined(__MRC__)
 void *alloca(unsigned);
 #  else
-char *alloca();
+void *alloca(size_t);
 #  endif
 # endif
 #endif