netatalk: needs dynamic library

netatalk: needs dynamic library

Even though netatalk checks if dlfcn.h is available and has some code
that supposedly copes with non-dynamic library cases, it in fact does
not build. Since there is probably little interest in using netatalk
in pure statically linked environments, let's simply require dynamic
library support.

Fixes:

  http://autobuild.buildroot.net/results/79e55e64885d64392407e830560175c556953a20/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>