opkg: re-enable usign support

The switch to cmake caused the -DHAVE_USIGN flag to get lost, disabling
compilation of the correspondinf support code.

Update to latest Git head which enables usign support by default.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

package/system/opkg/Makefile

@@ -15,8 +15,8 @@ PKG_RELEASE:=1
 
 PKG_SOURCE_PROTO:=git
 PKG_SOURCE_URL:=https://git.lede-project.org/project/opkg-lede.git
-PKG_SOURCE_DATE:=2017-02-18
-PKG_SOURCE_VERSION:=db3fc0ce879bfe784d32ffe5a70e057459039dd5
+PKG_SOURCE_DATE:=2017-02-20
+PKG_SOURCE_VERSION:=bec9804c73acf74ba112c9eec4ce7ba97e140c89
 PKG_MIRROR_HASH:=
 
 PKG_LICENSE:=GPL-2.0