mpg123: needs MMU

mpg123 needs MMU because the application that is built with this
package uses fork. Currently it is difficult to only build the
libraries for mpg123 so disabling the package all together when there
is no MMU support.

Note: mpg123 is an optional dependency of mpd but mpd already requires
BR2_USE_MMU so there is no need to add this as a dependency.

Fixes:
  http://autobuild.buildroot.net/results/5b0/5b053af566dd122ae7e58893e77d5d5f3070fb9e



Signed-off-by: Ryan Barnett <rjbarnet@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>