alsa-lib: fix build failures for nommu

Disable the pcm_shm plugin (aserver) and alisp for nommu systems since
they use fork()
Fixes
http://autobuild.buildroot.net/results/7e59e73e516579045538e7cc0efbc22744a45d75



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>