uci: fix variable declaration

Using := doesn't fly well when including other variables. In fact this
would cause the variable to be empty and break cloning of the git repo.

Fix: "c354c069b3 uci: fix Makefile formatting"

Signed-off-by: Paul Spooren <[email protected]>

package/system/uci/Makefile

@@ -11,7 +11,7 @@ include $(TOPDIR)/rules.mk
 PKG_NAME:=uci
 PKG_RELEASE:=1
 
-PKG_SOURCE_URL:=$(PROJECT_GIT)/project/uci.git
+PKG_SOURCE_URL=$(PROJECT_GIT)/project/uci.git
 PKG_SOURCE_PROTO:=git
 PKG_SOURCE_DATE:=2023-08-10
 PKG_SOURCE_VERSION:=5781664d5087ccc4b5ab58505883231212dbedbc