stream: stupid compilation workaround for win32

On win32, open() is a function-like macro. The line of code changed
with this commit accidentally expanded the macro. Prevent this macro
expansion. Not sure why that happened now. Since as far as I remember
system functions can be defined as macros, this affects in theory not
only win32.

stream/stream.c

@@ -312,7 +312,7 @@ static int open_internal(const stream_info_t *sinfo, struct stream *underlying,
         }
     }
 
-    int r = sinfo->open(s);
+    int r = (sinfo->open)(s);
     if (r != STREAM_OK) {
         talloc_free(s);
         return r;