Makefile: Add a disctclean target (simply calling clean)

We currently don't distribute anything that's not already in git, so
there's no difference between these two targets, (but debhelper wants
to be able to call distclean).

Makefile.local

@@ -138,6 +138,11 @@ DEPS := $(DEPS:%.cc=.deps/%.d)
 clean:
 	rm -f $(CLEAN); rm -rf .deps
 
+# We don't (yet) have any distributed files not in the upstream repository.
+# So distclean is currently identical to clean.
+.PHONY: distclean
+distclean: clean
+
 notmuch_client_srcs =		\
 	$(notmuch_compat_srcs)	\
 	debugger.c		\