dmm/bm52x: unbreak build when serial comm is not available

dmm/bm52x: unbreak build when serial comm is not available

Make another code path in the Brymen BM52x packet parser optional which
snuck in earlier and broke compilation when serial_comm dependencies are
not met.

Ideally the src/dmm/ parser code would not get built at all in that
case, but building a parser with reduced functionality and not using it
later because its callers are conditional is as good, and is what other
DMM packet parsers did in the past.

How to reproduce:

  $ configure --without-libserialport --without-libhidapi --without-libbluez
  $ make