knock: fix static build

Use pcap-config to determine link dependencies of libpcap.

Fixes:
http://autobuild.buildroot.net/results/1eb/1eb877ebd9469c9261bf114fc7733819ae3fd562/

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/knock/knock.mk

@@ -10,4 +10,8 @@ KNOCK_LICENSE = GPLv2+
 KNOCK_LICENSE_FILES = COPYING
 KNOCK_DEPENDENCIES = libpcap
 
+ifeq ($(BR2_PREFER_STATIC_LIB),y)
+KNOCK_CONF_OPT = LIBS="$(shell $(STAGING_DIR)/usr/bin/pcap-config --static --additional-libs)"
+endif
+
 $(eval $(autotools-package))