Cygwin is POSIX, don't say it isn't

More to the point, Cygwin is a POSIX API.  In our library, the use of
a POSIX API is marked by defining the macro OPENSSL_SYS_UNIX.
Therefore, that macro shouldn't be undefined when building for Cygwin.
Reviewed-by: NAndy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5060)

include/openssl/e_os2.h

@@ -49,7 +49,6 @@ extern "C" {
 #  define OPENSSL_SYS_WIN32_UWIN
 # else
 #  if defined(__CYGWIN__) || defined(OPENSSL_SYS_CYGWIN)
-#   undef OPENSSL_SYS_UNIX
 #   define OPENSSL_SYS_WIN32_CYGWIN
 #  else
 #   if defined(_WIN32) || defined(OPENSSL_SYS_WIN32)