package/qpid-proton: disable for static build only

qpid-proton always build a shared library "libqpid-proton.so"

Fixes:
http://autobuild.buildroot.net/results/bf6/bf6b9bcc0cead87c8fb12ec4c7f5ff73e6d61d3f/



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>