jack2: requires MMU support

jack2 uses fork(), so it's not available on non-MMU architectures.

Fixes:

  http://autobuild.buildroot.org/results/0d4/0d441bf1831b2a657b466e44c29423c45c07decd/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>