package/dhcp: fix build with zlib

internal bind is in version 9.11 and so doesn't support pkg-config
like bind 9.16 resulting in the following build failure since commit
0c8dd6ebd656e06cf99a63eb93343715f4853503:

configure: error: include/zlib.h not found.

Fixes:
 - http://autobuild.buildroot.org/results/7a5cdf30881d208807976cf98960c5fe2abfed50

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/dhcp/dhcp.mk

@@ -42,7 +42,7 @@ DHCP_CONF_OPTS = \
 	--with-relay6-pid-file=/var/run/dhcrelay6.pid
 
 ifeq ($(BR2_PACKAGE_ZLIB),y)
-DHCP_BIND_EXTRA_CONFIG += --with-zlib
+DHCP_BIND_EXTRA_CONFIG += --with-zlib=$(STAGING_DIR)/usr
 DHCP_DEPENDENCIES += zlib
 else
 DHCP_BIND_EXTRA_CONFIG += --without-zlib