packaging: fix "make centos{5,6,7}"

Without this change, make centos{5,6,7} fails with error:
    cp: cannot stat '<basedir>/rpmbuild/RPMS/*/*': No such file or directory

This change is the exact equivalent of bc4bbc42899a (which applied to Fedora).

contrib/packaging/Makefile

@@ -120,8 +120,8 b' define centos_targets'
 centos$(1):
 	mkdir -p $$(HGROOT)/packages/centos$(1)
 	./buildrpm $$(if $$(filter $(1),$$(CENTOS_WITH_PYTHON_RELEASES)),--withpython)
-	cp $$(HGROOT)/rpmbuild/RPMS/*/* $$(HGROOT)/packages/centos$(1)
-	cp $$(HGROOT)/rpmbuild/SRPMS/* $$(HGROOT)/packages/centos$(1)
+	cp $$(HGROOT)/contrib/packaging/rpmbuild/RPMS/*/* $$(HGROOT)/packages/centos$(1)
+	cp $$(HGROOT)/contrib/packaging/rpmbuild/SRPMS/* $$(HGROOT)/packages/centos$(1)
 
 .PHONY: docker-centos$(1)
 docker-centos$(1):