crt: gccmain: Remove includes

Include headers are not needed. This makes the gccmain.c indepedent of the
mingw-w64 header files, like the libgcc's __main implementation.

Signed-off-by: LIU Hao <lh_mouse@126.com>

mingw-w64-crt/crt/gccmain.c

diff --git a/mingw-w64-crt/crt/gccmain.c b/mingw-w64-crt/crt/gccmain.c
index e2eb08b..7d8c0e7 100644
--- a/mingw-w64-crt/crt/gccmain.c
+++ b/mingw-w64-crt/crt/gccmain.c
@@ -4,10 +4,7 @@
  * No warranty is given; refer to the file DISCLAIMER.PD within this package.
  */
 
-#include <windows.h>
-#include <stdlib.h>
-#include <setjmp.h>
-
+int __cdecl atexit (void (__cdecl *)(void));
 typedef void (*func_ptr) (void);
 extern func_ptr __CTOR_LIST__[];
 extern func_ptr __DTOR_LIST__[];