package/alsa-lib: add BR2_PACKAGE_ALSA_LIB_UCM

package/alsa-lib: add BR2_PACKAGE_ALSA_LIB_UCM

ucm needs mixer since version 1.2.1 and
https://github.com/alsa-project/alsa-lib/commit/5c88b183718ce3df11bc4753c76d95f3e9de6c7e

So add a BR2_PACKAGE_ALSA_LIB_UCM option that selects
BR2_PACKAGE_ALSA_LIB_MIXER and selects this new option in
BR2_PACKAGE_ALSA_UTILS_ALSAUCM to avoid a build failure

Fixes:
 - http://autobuild.buildroot.org/results/aaa615d4e38d100bb9001ecdf7357e755ef42a87

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>