Add some missing 'const' in version.c's string data.

I can't believe this codebase is around 20 years old and has had
multiple giant const-fixing patches, and yet there are _still_ things
that should have been const for years and aren't.

windows/winpgen.c

@@ -134,7 +134,7 @@ static void progress_update(void *param, int action, int phase, int iprogress)
     }
 }
 
-extern char ver[];
+extern const char ver[];
 
 struct PassphraseProcStruct {
     char **passphrase;