Revert "Consider winsdk_samples a system library."

This reverts commit 6e8a05d4d6909c684bcf05114ab6d27ffdf846fa.

Reason for revert: clang-cl doesn't have -isystem, but /imsvc does
what we need.

Change-Id: Ie6ce521effc67491586f2f16ea3e03a6c78b69c0
TBR=phoglund@webrtc.org
Reviewed-on: https://webrtc-review.googlesource.com/76520
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>