vim-patch:8.1.0941: macros for MS-Windows are inconsistent (#20215)

Problem:    Macros for MS-Windows are inconsistent, using "32", "3264 and
            others.
Solution:   Use MSWIN for all MS-Windows builds.  Use FEAT_GUI_MSWIN for the
            GUI build. (Hirohito Higashi, closes vim/vim#3932)
4f97475d32

src/nvim/api/vim.c

@@ -1882,7 +1882,7 @@ Object nvim_get_proc(Integer pid, Error *err)
     api_set_error(err, kErrorTypeException, "Invalid pid: %" PRId64, pid);
     return NIL;
   }
-#ifdef WIN32
+#ifdef MSWIN
   rvobj.data.dictionary = os_proc_info((int)pid);
   if (rvobj.data.dictionary.size == 0) {  // Process not found.
     return NIL;