libpcap: handle bluetooth explicitly

If bluez_utils is around then add it to dependencies so as to use it,
but if it's not (for instance bluez5_utils is around) then disable it
since the bluez5 API isn't compatible. Fixes:
http://autobuild.buildroot.net/results/6ca/6caeb7f41ac5e14dbee49f87511af3351f6013f1/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>