bpf: fix build with some libpcap version on FreeBSD

bpf: fix build with some libpcap version on FreeBSD

This is something caught in UNH FreeBSD env.

For some reason [1], the pcap/bpf.h header started to define _BPF_H_.

It happens that the bpf_impl.h internal DPDK header uses this define as
an internal guard.
This triggers a build failure in bpf_convert.c which can't find
RTE_BPF_LOG macro.

Fix the include guard to use the filename and remove _.

1: https://github.com/the-tcpdump-group/libpcap/pull/1074

Fixes: 94972f35a02e ("bpf: add BPF loading and execution framework")
Cc: stable@dpdk.org
Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>