Fix compiler warning in reliable.c with --disable-debug

Use the easy way out. Using pre-compiler to completely
avoid n_active seems like overkill.

Change-Id: Icad1a52d14311a6f06bda081cab2f4bded8d47ed
Signed-off-by: Frank Lichtenheld <[email protected]>
Acked-by: Arne Schwabe <[email protected]>
Message-Id: <[email protected]>
URL: https://www.mail-archive.com/[email protected]/msg32107.html
Signed-off-by: Gert Doering <[email protected]>
(cherry picked from commit fcd8f0f)

Author: flichtenheld

src/openvpn/reliable.c

@@ -649,6 +649,7 @@ reliable_can_send(const struct reliable *rel)
             }
         }
     }
+    (void)n_active; /* dmsg might not generate code */
     dmsg(D_REL_DEBUG, "ACK reliable_can_send active=%d current=%d : %s",
          n_active,
          n_current,