main: Restore public domain comment in Windows code.

This, strictly speaking, doesn't _need_ to be public domain any more, since
SDL3 no longer inserts this code into apps via a static SDL_main library; it's
just part of the normal zlib-licensed SDL library and called into through a
well-defined entry point at startup, now.

But removing a public domain notice isn't the done thing, so we'll leave it in.

(To a related point: even if others have rewritten good portions of this code,
they're doing it under the assumption that this specific file has a public
domain license.)

src/main/windows/SDL_sysmain_runapp.c

@@ -24,6 +24,9 @@
 
 #include "../../core/windows/SDL_windows.h"
 
+/* Win32-specific SDL_RunApp(), which does most of the SDL_main work,
+  based on SDL_windows_main.c, placed in the public domain by Sam Lantinga  4/13/98 */
+
 #include <shellapi.h> // CommandLineToArgvW()
 
 static int OutOfMemory(void)