samples/subsys/audio/sof: build with CONFIG_SPEED_OPTIMIZATIONS

The inner loops of SOF audio algorithms benefit from
compiler optimizations. On Intel cAVS platforms, it was noted
zero-overhead loops ("loop" family of instructions) is not
used for fixed-size inner loops when using -Os (optimize for
size) and the XCC xtensa toolchain.

Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>