package/erlang: disable for static builds

erlang wants to build shared libs, always.

So, disable erlang when we're building static-only.

Fixes:
    http://autobuild.buildroot.net/results/1ce/1ceaa6925d1ded236d01ae9feb31f5f915f6b51a/



Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Frank Hunleth <fhunleth@troodon-software.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>