Fix makefile function typo

Makefile

@@ -6,6 +6,8 @@ define rpm_eval
 	shell rpm --eval '%{$(1)}'
 endef
 
+TARBALL_PATH = $(call rpm_eval _sourcedir)/$(NAME)-$(VERSION).tar.gz
+
 .PHONY: name
 name:
 	@echo "$(NAME)"
@@ -14,7 +16,7 @@ name:
 version:
 	@echo "$(VERSION)"
 
-$(RPM_SOURCEDIR)/$(NAME)-%.tar.gz: *
+$(TARBALL_PATH): *
 	git archive --format=tar.gz \
 	            --output="$@" \
 	            --prefix="$(NAME)-$(VERSION)/" \
@@ -22,7 +24,7 @@ $(RPM_SOURCEDIR)/$(NAME)-%.tar.gz: *
 	            "$(BRANCH)"
 
 .PHONY: tarball
-tarball: $(RPM_SOURCEDIR)/$(NAME)-$(VERSION).tar.gz
+tarball: $(TARBALL_PATH)
 
 .PHONY: install
 install: