[boost] [fiber] PR: Replace #include "Windows.h" by "windows.h"

Hi, A #include "Windows.h" causes troubles when cross-compiling on Linux in C++14 mode with MinGW-W64 with Posix threads. Can we change it to "windows.h"? Here is a PR: https://github.com/boostorg/fiber/pull/134 Thanks, Frédéric