Fix building for Win64 target

* random/random-csprng.c [HAVE_W32_SYSTEM]: Include <windows.h>.
* tests/bench-slope.c [_WIN32]: Include <windows.h>.

Removal of <winsock2.h> include from 'gcrypt.h' broke building
random-csprng.c and bench-slope.c on x86_64-w64-mingw32 target.

• GnuPG-bug-id: T5637
• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>