luci-app-mosdns: fix an error reported when using imageBuilder

luci-app-mosdns/Makefile

@@ -17,6 +17,7 @@ endef
 
 define Package/$(PKG_NAME)/postinst
 #!/bin/sh
+[ ! -f /etc/openwrt_release ] && exit 0
 [ -n "${IPKG_INSTROOT}" ] || {
 	sysctl -p /etc/sysctl.d/20-mosdns-buffer-increase.conf
 	exit 0

luci-app-mosdns/root/etc/init.d/mosdns

@@ -20,6 +20,8 @@
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
 #
 
+[ ! -f /etc/openwrt_release ] && exit 0
+
 START=99
 USE_PROCD=1