ALSA: Export snd_pcm_constraint_mask64()

Allow modules to use it, fixing a build failure when the newly added
ADAU1977 driver is built as a module.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Takashi Iwai <tiwai@suse.de>