MSVC: Fix include files

Error message:

  gost12sum.c(13,10): fatal error C1083: Cannot open include file: 'unistd.h': No such file or directory
  gost12sum.c(80,23): warning C4013: 'getopt' undefined; assuming extern returning int
  test_keyexpimp.c(7,10): fatal error C1083: Cannot open include file: 'arpa/inet.h': No such file or directory

Signed-off-by: Vitaly Chikunov <vt@altlinux.org>