libpcap: fix build failure related to libnl3

When libpcap detects that libnl3 is available, it hardcodes the fact
of using -I/usr/include/libnl3, which is completely wrong in
cross-compilation.

This commit fixes that, by removing this hardcoded include path, and
by passing an appropriate -I option from libpcap.mk.

The original issue has been reported upstream at
https://github.com/the-tcpdump-group/libpcap/issues/305.

Fixes:

  http://autobuild.buildroot.net/results/b65/b6585bf2985a188e797e5a6b71a35c461421e3b4/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>