bpf: fix convert API when libpcap missing

bpf: fix convert API when libpcap missing

rte_bpf_convert() implementation depends on libpcap.
Right now it is defined only when this library is installed and
RTE_PORT_PCAP is defined.
Fix that by providing for such case stub rte_bpf_convert()
implementation that will always return an error.
To draw user attention, if proper implementation is disabled,
warning will be thrown at meson configure stage.
Also move stub for another function (rte_bpf_elf_load) into
the same place (bpf_stub.c).

Fixes: 2eccf6afbea9 ("bpf: add function to convert classic BPF to DPDK BPF")
Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>