Changes to build 'winhttp.c'

* src/libgit2/transports/winhttp.c: Changes to build and fix compiler
  warnings:

   - Define __CRT_STRSAFE_IMPL to fix duplicate extern error from strsafe.h
   - Add CLSID_InternetSecurityManager
   - Add IID_IInternetSecurityManager
   - Remove #pragma comment(lib, "winhttp")
   - Remove #pragma comment(lib, "rpcrt4")
   - Remove unused variable 'default_port' in function 'winhttp_connect'
   - Remove unused variable 't' in function 'winhttp_stream_write_single'
   - Remove unused variable 't' in function 'winhttp_stream_write_buffered'
   - Remove unused variable 't' in function 'winhttp_stream_write_chunked'
   - Remove defined but unused 'prefix_http'
   - Use format specifier PRIxZ in git_buf_printf in function 'write_chunk'
   - Initialize 'wide_len' to 0 in function 'apply_basic_credential'

* misc/winhttp-build-with-mingw-w64.patch: Update

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