opkg: fix PKG_CONFIG_DEPENDS to include version.mk entries

Including version.mk sets PKG_CONFIG_DEPENDS to config entries used for
VERSION_SED command. We should keep these configs to make sure package
gets refreshed when needed.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>

package/system/opkg/Makefile

@@ -23,7 +23,8 @@ PKG_LICENSE_FILES:=COPYING
 
 PKG_MAINTAINER:=Jo-Philipp Wich <jo@mein.io>
 
-PKG_CONFIG_DEPENDS := \
+# Extend depends from version.mk
+PKG_CONFIG_DEPENDS += \
 	CONFIG_SIGNED_PACKAGES \
 	CONFIG_TARGET_INIT_PATH