Revert "Use libmingwex for gmtime_s"

The new version of mingw has moved all of time_s.h into time.h.

This reverts commit 1f6fdd5a306c0a08f5dcf7ef6696c2efe4839882.

Bug: http://b/26523949
Change-Id: I58f05135ac44fa053133e0cfe999054b2694b682
1 file changed