Fixing "no driver found for eth_pcap0"

I was getting a "no driver found for eth_pcap0" error message whenever I wanted to use an "eth_pcap" virtual device with my DPDK application. But the DPDK sample app "test-pmd" worked just fine with those. So I took a look at its Makefile and found out adding this line to my Makefile fixes the issue:

DEPDIRS-y += lib drivers