alsa-utils: amidi option needs seq support in alsa-lib

Fixes:
http://autobuild.buildroot.net/results/5347b57d23b44e1b011a499875ce638ed654418a/



Otherwise the rawmidi_virt.c file doesn't get built, breaking static
linking.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>