netsniff-ng trafgen: Find libnl-3.0 and libnl-genl-3.0 using pkg-config

Call pkg-config --cflags and pkg-config --libs to find correct CFLAGS
and LDFLAGS respectively.

Signed-off-by: Peter Stuge <peter@stuge.se>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>