ASoC: pxa: select SND_PXA2XX_LIB for drivers that depend on it

Commit d767d3ce ("ASoC: pxa: provide PCM ops for ssp, i2s and ac97
components") created a build-time dependency to SND_PXA2XX_LIB but
missed to reflect that in Kconfig.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: Mark Brown <broonie@kernel.org>