package/ngrep: fix static build

package/ngrep: fix static build

Use pcap-config to list optional libpcap dependencies that we need to list
when building statically.

Inspired by Baruch Siach
http://git.buildroot.net/buildroot/commit/package/dhcpdump/dhcpdump.mk?id=429f4415cd153c6809394a8b3245d4d15bba3ec3

Fixes
http://autobuild.buildroot.net/results/3c5/3c584b850cabebcf93dfd61c59e28988165a41c4/
http://autobuild.buildroot.net/results/938/93842b54767cca51c68ad33ddc93ec58d70602c8/
http://autobuild.buildroot.net/results/0ae/0ae4adec7fe66f819c7b3dabf13648fb29247de6/
http://autobuild.buildroot.net/results/9c2/9c203bc6ca91da68c9e7335611c1fb56224380bc/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>