src: define getpid() based on OS

94b9948 added unistd.h to src/env.cc in order to use getpid().
However, this doesn't exist on Windows. This commit conditionally
defines getpid() based on the OS.

Fixes: nodejs#4145
PR-URL: nodejs#4146
Reviewed-By: Brian White <mscdex@mscdex.net>

src/env.cc

@@ -1,7 +1,13 @@
 #include "env.h"
 #include "env-inl.h"
 #include "v8.h"
-#include "unistd.h"
+
+#if defined(_MSC_VER)
+#define getpid GetCurrentProcessId
+#else
+#include <unistd.h>
+#endif
+
 #include <stdio.h>
 
 namespace node {