msvc: do not re-declare the timespec struct

VS2015's headers already declare that struct.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.h

@@ -352,11 +352,13 @@ static inline int getrlimit(int resource, struct rlimit *rlp)
 #ifndef __MINGW64_VERSION_MAJOR
 #define off_t off64_t
 #define lseek _lseeki64
+#ifndef _MSC_VER
 struct timespec {
 	time_t tv_sec;
 	long tv_nsec;
 };
 #endif
+#endif
 
 struct mingw_stat {
     _dev_t st_dev;