libcoap: needs host-pkgconf

The libcoap configure script uses pkg-config.

Fixes:
http://autobuild.buildroot.net/results/746/746c6dcbf3f941c7baa5b382bd264d830d839be1/
http://autobuild.buildroot.net/results/d98/d9840a22ae5eb6572b6641b93f8beadc27d2abb5/
http://autobuild.buildroot.net/results/905/905f122213ad9623ea5dd9d32c352efd151319f3/

Cc: Joris Lijssens <joris.lijssens@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libcoap/libcoap.mk

@@ -9,6 +9,7 @@ LIBCOAP_SITE = $(call github,obgm,libcoap,$(LIBCOAP_VERSION))
 LIBCOAP_INSTALL_STAGING = YES
 LIBCOAP_LICENSE = GPL-2.0+ or BSD-2-Clause
 LIBCOAP_LICENSE_FILES = COPYING LICENSE.GPL LICENSE.BSD
+LIBCOAP_DEPENDENCIES = host-pkgconf
 LIBCOAP_CONF_OPTS = --disable-examples
 LIBCOAP_AUTORECONF = YES