[benchmark] Lowercase windows specific includes

The windows SDK headers don't have self-consistent casing anyway,
so we consistently use lowercase for these in other places, in order
to fix crosscompilation with mingw headers.

This applies an upstream commit:
https://github.com/google/benchmark/commit/52613079824ac58d06c070aa9fbbb186a5859e2c

Differential Revision: https://reviews.llvm.org/D52181

llvm-svn: 342450