package/bind: fix libxml2 build

Fix the following build failure with libxml2 raised since bump to
version 9.16.26 in commit 8adeaec8afacbc680edebae91524f6144926ac92:

configure: error: Specifying libxml2 installation path is not supported, adjust PKG_CONFIG_PATH instead

Fixes:
 - http://autobuild.buildroot.org/results/648d29e4cfa6a40bb6e54793c044e9c834f03a1b

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/bind/bind.mk

@@ -67,7 +67,7 @@ BIND_CONF_OPTS += --with-gssapi=no
 endif
 
 ifeq ($(BR2_PACKAGE_LIBXML2),y)
-BIND_CONF_OPTS += --with-libxml2=$(STAGING_DIR)/usr
+BIND_CONF_OPTS += --with-libxml2
 BIND_DEPENDENCIES += libxml2
 else
 BIND_CONF_OPTS += --with-libxml2=no