nodogsplash: check if TrafficControl is enabled before testing imq module requirement.

This avoids failing to start in current OpenWrt Attitude Adjustment and trunk,
which lack IMQ support (and WONTFIX). Suggested in the issue discussed in:
http://ml.ninux.org/pipermail/nodogsplash/2013-May/000009.html

Signed-off-by: Gui Iribarren <gui@altermundi.net>