flowtop: Include netlink dissector to fix build temporarily

flowtop links in the dissector modules just for the TC/UDP port lookup
feature. Since dissector.h now includes libnl headers, we also need to
include the libnl CFLAGS for flowtop build and thus also make it depend
on libnl.

The real fix for this in the future would be to extract the port lookup
from dissector_eth.c and the just link in this module into flowtop
instead of the entire dissector stuff.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>