opkg: Fix download over git

Set the PKG_SOURCE_URL using a lazy set to allow evaluating
$(PROJECT_GIT) later. Without this change PKG_SOURCE_URL is evaluated
immediately, before PROJECT_GIT is defined and the download over git is
not working.

Fixes: 6687a2483a09 ("opkg: use $(PROJECT_GIT), $(AUTORELEASE) and SPDX")
Signed-off-by: Hauke Mehrtens <[email protected]>

package/system/opkg/Makefile

@@ -11,7 +11,7 @@ PKG_RELEASE:=$(AUTORELEASE)
 PKG_FLAGS:=essential
 
 PKG_SOURCE_PROTO:=git
-PKG_SOURCE_URL:=$(PROJECT_GIT)/project/opkg-lede.git
+PKG_SOURCE_URL=$(PROJECT_GIT)/project/opkg-lede.git
 PKG_SOURCE_DATE:=2021-03-15
 PKG_SOURCE_VERSION:=5936c4f9660248284e8a9b040ea3153d3ea888de
 PKG_MIRROR_HASH:=b873c209baaf4f150c89646d58e4a0072f807d24b02c320ab8c7ae9180c13240