add hard dependency on depclean

Currently executing `make distclean publish` doesn't work due to parallel build.
Workaround: `make distclean; make publish`

Discussions: 
http://gnu-make.2324884.n4.nabble.com/order-only-prerequisites-don-t-behave-as-I-d-have-expected-after-reading-the-documentation-tt3787.html#a15681
