Redefine getpid() -> _getpid() only for MSVC

This was introduced in 814b5133e for MSVC. C++Builder doesn't need it.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/16192)

apps/lib/s_socket.c

@@ -27,12 +27,12 @@ typedef unsigned int u_int;
 #endif
 
 #ifdef _WIN32
-/*
- * With MSVC, certain POSIX functions have been renamed to have an underscore
- * prefix.
- */
 # include <process.h>
-# define getpid _getpid
+
+/* MSVC renamed some POSIX functions to have an underscore prefix. */
+# ifdef _MSC_VER
+#  define getpid _getpid
+# endif
 #endif
 
 #ifndef OPENSSL_NO_SOCK