[mythtv] Latest CVS compile error in signalmonitor.cpp when not using DVB

[Jack Porter]

I needed to add #ifdef USING_DVB/#endif around #include 
"dvbsignalmonitor.h" and #include "dvbchannel.h" at the top of 
signalmonitor.cpp.

I don't use DVB and my kernel sources don't have support, so I was 
getting the #error "DVB driver includes with API version 3 not found!" 
from dvbtypes.

Please flame me if I'm doing something wrong :-)

- Jack