ltris: correct audio configure options

Fixes:
http://autobuild.buildroot.net/results/555/555bfc97b39eb9b0f1e516ffb7ae375ca2c43d5f/



The configure options to enable/disable sound through sdl_mixer are called
--enable/--disable-sound, not --enable/--disable-audio.

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