package/poco: disable for static build

poco requires dlopen(). There is a --no-sharedlibs option that is
supposed to support building in a statically linked environment, but
it doesn't do anything.

Fixes:

  http://autobuild.buildroot.net/results/952/952f05efd245ba59991f3c5be02a0572e8b9e544/



Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>