swconfig: Link with libubox

Fixes linking failures observed with external toolchains:

/home/florian/dev/toolchains/stbgcc-4.8-1.5/bin/../lib/gcc/mipsel-linux-gnu/4.8.5/../../../../mipsel-linux-gnu/bin/ld:
warning: libubox.so, needed by
/home/florian/dev/openwrt/trunk/staging_dir/target-mipsel-unknown-linux-gnu_glibc/usr/lib/libuci.so,
not found (try using -rpath or -rpath-link)
/home/florian/dev/openwrt/trunk/staging_dir/target-mipsel-unknown-linux-gnu_glibc/usr/lib/libuci.so:
undefined reference to `blobmsg_open_nested'
/home/florian/dev/openwrt/trunk/staging_dir/target-mipsel-unknown-linux-gnu_glibc/usr/lib/libuci.so:
undefined reference to `blobmsg_parse'
/home/florian/dev/openwrt/trunk/staging_dir/target-mipsel-unknown-linux-gnu_glibc/usr/lib/libuci.so:
undefined reference to `blob_nest_end'
/home/florian/dev/openwrt/trunk/staging_dir/target-mipsel-unknown-linux-gnu_glibc/usr/lib/libuci.so:
undefined reference to `blobmsg_add_field'

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

package/network/config/swconfig/Makefile

@@ -34,7 +34,7 @@ define Build/Compile
 	CFLAGS="$(TARGET_CPPFLAGS) $(TARGET_CFLAGS)" \
 	$(MAKE) -C $(PKG_BUILD_DIR) \
 		$(TARGET_CONFIGURE_OPTS) \
-		LIBS="$(TARGET_LDFLAGS) -lnl-tiny -lm -luci"
+		LIBS="$(TARGET_LDFLAGS) -lnl-tiny -lm -luci -lubox"
 endef
 
 define Build/InstallDev