libubox: set RPATH for host build

This is required for programs that indirectly link libjson-c through the
libubox blobmsg_json library.

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

package/libs/libubox/Makefile

@@ -97,6 +97,9 @@ endef
 CMAKE_HOST_OPTIONS += \
 	-DBUILD_LUA=OFF \
 	-DBUILD_EXAMPLES=OFF \
+	-DCMAKE_SKIP_RPATH=FALSE \
+	-DCMAKE_MACOSX_RPATH=1 \
+	-DCMAKE_INSTALL_RPATH="${STAGING_DIR_HOST}/lib" \
 
 $(eval $(call BuildPackage,libubox))
 $(eval $(call BuildPackage,libblobmsg-json))