Update file_dialog_open.cpp

line 35 added defined

Former-commit-id: 77f9ce765ad49249ab48b39d0ccf227cb5b5b161

include/igl/file_dialog_open.cpp

@@ -32,7 +32,7 @@ IGL_INLINE std::string igl::file_dialog_open()
   while ( fgets(buffer, FILE_DIALOG_MAX_BUFFER, output) != NULL )
   {
   }
-#elif _WIN32
+#elif defined _WIN32
   
   // Use native windows file dialog box
   // (code contributed by Tino Weinkauf)