vendor/makefile: fix build error for parallel builds

vendor/ubuntu/makefile

@@ -5,7 +5,7 @@ SUBDIR=vendor/ubuntu
 # Bring in the default rules
 include ../../buildlib/defaults.mak
 
-doc: sources.list
+doc binary: sources.list
 
 sources.list: sources.list.in ../../doc/apt-verbatim.ent
 	sed -e 's#&ubuntu-codename;#$(shell ../getinfo ubuntu-codename)#g' $< > $@