correct the way of testing file mode on Windows

src/google/protobuf/compiler/importer.cc

diff --git a/src/google/protobuf/compiler/importer.cc b/src/google/protobuf/compiler/importer.cc
index 0e01334..63a8704 100644
--- a/src/google/protobuf/compiler/importer.cc
+++ b/src/google/protobuf/compiler/importer.cc
@@ -495,7 +495,11 @@
   do {
     ret = stat(filename.c_str(), &sb);
   } while (ret != 0 && errno == EINTR);
+#ifdef _WIN32
+  if ((sb.st_mode & _S_IFMT) == _S_IFREG) {
+#else
   if (!S_ISREG(sb.st_mode)) {
+#endif
     last_error_message_ = "Input file is not a regular file.";
     return NULL;
   }