alsa-utils: disable on no-mmu platforms (fork)

alsa-utils uses fork which is not available on targets without MMU support.
It seems to be possible to replace fork with vfork in alsa-utils, but we do
not like to carry such patches in buildroot without them being accepted
upstream.

Fixes
http://autobuild.buildroot.org/results/9f8/9f8e572c9f1c677155cc7d1828371bcf572ff878



Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>