Changes to build 'winhttp.c'

* src/libgit2/transports/winhttp.c: Changes to build with mingw-w64,
  - Remove #include <strsafe.h>
  - Add CLSID_InternetSecurityManager
  - Add IID_IInternetSecurityManager

* misc/winhttp-build-with-mingw-w64.patch: Patch to apply changes.

* Makefile: Update target sync_libgit2 to apply patch

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>