use the compiler-defined _WIN32 define

Summary:
WIN32 is defined by windows.h. _WIN32 is defined by the compiler. The
latter is more reliable under refactoring.

Reviewed By: xavierd

Differential Revision: D47528610

fbshipit-source-id: 56318741cdf9d8e234c0b8d55aeb7f96e7d5efaa

watchman/cmds/info.cpp

@@ -140,7 +140,7 @@ static UntypedResponse cmd_get_sockname(Client*, const json_ref&) {
         "unix_domain", w_string_to_json(w_string::build(get_unix_sock_name())));
   }
 
-#ifdef WIN32
+#ifdef _WIN32
   if (!disable_named_pipe) {
     resp.set(
         "named_pipe",