include: Avoid _onexit_t redefinition.

include/msvcrt/corecrt_startup.h

@@ -32,7 +32,10 @@ typedef struct _onexit_table_t {
     _PVFV *_end;
 } _onexit_table_t;
 
+#ifndef _CRT_ONEXIT_T_DEFINED
+#define _CRT_ONEXIT_T_DEFINED
 typedef int (__cdecl *_onexit_t)(void);
+#endif
 
 struct _exception;
 typedef int (__cdecl *_UserMathErrorFunctionPointer)(struct _exception *);

include/msvcrt/stdlib.h

@@ -136,7 +136,10 @@ errno_t       __cdecl _get_errno(int*);
 errno_t       __cdecl _set_doserrno(int);
 errno_t       __cdecl _set_errno(int);
 
+#ifndef _CRT_ONEXIT_T_DEFINED
+#define _CRT_ONEXIT_T_DEFINED
 typedef int (__cdecl *_onexit_t)(void);
+#endif
 
 
 int           __cdecl _atodbl(_CRT_DOUBLE*,char*);