Fix Visual Studio warning

src/civetweb.c

@@ -407,6 +407,8 @@ static void path_to_unicode(const struct mg_connection *conn,
 /* All file operations need to be rewritten to solve #246. */
 #include "file_ops.inl"
 
+struct mg_file;
+
 static const char *
 mg_fgets(char *buf, size_t size, struct mg_file *filep, char **p);