utils: nfsynproxy: fix error while compiling the BPF filter

Fix the following error while running nfsynproxy here:

 pcap_compile: not-yet-activated pcap_t passed to pcap_compile

According to what I have read, we have to compile the filter
once the pcap_t handle has been activated.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>