package/asterisk: disable spandsp

asterisk fails to build with spandsp since commit 36c983bf
(package/spandsp: bump version to 3.0.0), so disable it.

Fixes:
 - http://autobuild.buildroot.org/results/c984738cad4186edb12a62b8e74115e68efe35f0



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>