Use lowercase windows.h for building on Linux with MinGW

[aixxe]

A few very minor changes to make life a bit easier for anyone cross-compiling under Linux.

Here's a minimal example to confirm the build error with Docker:

docker run --rm -it archlinux:base-devel /bin/bash
pacman --sync --refresh --noconfirm --needed git ninja cmake mingw-w64 python
git clone https://github.com/cursey/safetyhook.git
cmake -S safetyhook/ -G Ninja -B safetyhook_build/ \
  -DCMAKE_SYSTEM_NAME=Windows \
  -DCMAKE_SYSTEM_PROCESSOR=x86_64 \
  -DCMAKE_C_COMPILER=/usr/bin/x86_64-w64-mingw32-gcc \
  -DCMAKE_CXX_COMPILER=/usr/bin/x86_64-w64-mingw32-g++ \
  -DCMAKE_FIND_ROOT_PATH=/usr/x86_64-w64-mingw32 \
  -DCMAKE_BUILD_TYPE=Release \
  -DSAFETYHOOK_FETCH_ZYDIS=ON \
  -DSAFETYHOOK_BUILD_TESTS=OFF \
  -DSAFETYHOOK_AMALGAMATE=ON
cmake --build safetyhook_build/

The PR changes can be tested by running these additional two commands:

cd safetyhook
curl https://patch-diff.githubusercontent.com/raw/cursey/safetyhook/pull/44.patch | git apply
cmake --build ../safetyhook_build/

I ran into some other errors with xbyak when building with tests, so these changes will only cover building the library itself.

[cursey]

While this fixes cross compilation in the case of mingw, it creates issues for cross compilation using something like clang (or clang-cl) utilizing the real Windows SDK.

I think I'd prefer if the #include <Windows.h> were wrapped in a __has_include(...) similar to how Zydis.h is included. See:

safetyhook/src/inline_hook.cpp

Lines 5 to 11 in d952426

	#if __has_include(<Zydis/Zydis.h>)
	#include <Zydis/Zydis.h>
	#elif __has_include(<Zydis.h>)
	#include <Zydis.h>
	#else
	#error "Zydis not found"
	#endif

Something like:

#if __has_include(<Windows.h>)
#include <Windows.h>
#elif __has_include(<windows.h>)
#include <windows.h>
#else
#error "Windows.h not found"
#endif

[aixxe]

Thanks for reviewing! I've made those changes just now.

[cursey]

LGTM