Fix warning.

In DVBAPI < 505 there were SYS_DVBC_ANNEX_AC delivery system which was later
split into several others.

This fixes the following warning when using older kernel headers.

  dvb.c: In function 'FrontendInfo':
  dvb.c:810:9: warning: enumeration value 'SYS_DVBC_ANNEX_AC' not handled in switch [-Wswitch]