Lowercase some Windows headers.

MinGW on Linux needs lowercase include files. On Windows this doesn't
matter since the filesystems are case-insensitive, but building
BoringSSL on Linux with MinGW has case-sensitive filesystems.

Change-Id: Id9c120d819071b041341fbb978352812d6d073bc
Reviewed-on: https://boringssl-review.googlesource.com/4090
Reviewed-by: Adam Langley <agl@google.com>
12 files changed