include: remove unnecessary stuff on windows

Our definition of struct timespec used to cause problems with
certain versions of mingw-rt. However, as it turns out, we don't
actually need those definitions and prototypes, so remove them.

Fixes #9472.

Change-Id: Ie0880f0d58be112625140f73d0bed71f98b7cf05
Reviewed-on: https://go-review.googlesource.com/2236


Reviewed-by: Alex Brainman <alex.brainman@gmail.com>