mingw: avoid const warning

Fix const warnings in http-fetch.c and remote-curl.c main() where is
argv declared as const.

The fix should work for all future declarations of main, no matter
whether the second parameter's type is "char**", "const char**", or
"char *[]".

Signed-off-by: Stepan Kasal <kasal@ucw.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.h

@@ -372,7 +372,7 @@ static int mingw_main(c,v); \
 int main(int argc, char **argv) \
 { \
 	mingw_startup(); \
-	return mingw_main(__argc, __argv); \
+	return mingw_main(__argc, (void *)__argv); \
 } \
 static int mingw_main(c,v)