package/dhcp: disable backtrace on internal bind

Like was done in 700674b45c86 (package/bind: disable backtrace support)
for the up-to-date, official, upstream bind, also disable backtrace on
dhcp's internal bind to avoid the following build failure since commit
0c8dd6ebd656 (package/dhcp: use internal bind):

/nvmedata/autobuild/instance-15/output-1/host/lib/gcc/armeb-buildroot-linux-uclibcgnueabi/10.3.0/../../../../armeb-buildroot-linux-uclibcgnueabi/bin/ld: /nvmedata/autobuild/instance-15/output-1/build/dhcp-4.4.3/bind/bind-9.11.36/lib/isc/.libs/libisc.so: undefined reference to `_Unwind_GetIP'

Fixes:
 - http://autobuild.buildroot.org/results/074786f3f1e7ffc858dcb1de1855ee138793869e

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/dhcp/dhcp.mk

@@ -36,7 +36,8 @@ DHCP_BIND_EXTRA_CONFIG = \
 	BUILD_CFLAGS='$(HOST_CFLAGS)' \
 	BUILD_CPPFLAGS='$(HOST_CPPFLAGS)' \
 	BUILD_LDFLAGS='$(HOST_LDFLAGS)' \
-	RANLIB='$(TARGET_RANLIB)'
+	RANLIB='$(TARGET_RANLIB)' \
+	--disable-backtrace
 
 DHCP_CONF_ENV += ac_cv_prog_AWK=$(HOST_DIR)/bin/gawk