net/pcap: fix cross compilation with meson

The detection of pcap as a dependency involves invoking pcap-config to get
parameters - something not possible in a cross-compilation environment.
Therefore we need to just look for the presence of the library in a
cross-compilation environment and assume if the library is present we can
compile and link against it.

Fixes: efd5d1a8d8dd ("drivers/net: build some vdev PMDs with meson")

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Luca Boccassi <bluca@debian.org>