Fix how windows.h is included in our files.

To prevent bugs with stitching system headers inside Catch,
the proxy header is responsible for guarding against inclusion
on Linux, rather than the includers.

Might be related to #1197

include/internal/catch_session.cpp

@@ -19,6 +19,7 @@
 #include "catch_startup_exception_registry.h"
 #include "catch_text.h"
 #include "catch_stream.h"
+#include "catch_windows_h_proxy.h"
 
 #include <cstdlib>
 #include <iomanip>