src/ucm/main.c: fix build without mixer

src/ucm/main.c: fix build without mixer

Commit 4ce38a5ff466d18039b2606938f866ea3a6c9f3c breaks the build without
mixer on:

  CCLD     libasound.la
/home/buildroot/autobuild/instance-1/output-1/host/lib/gcc/xtensa-buildroot-linux-uclibc/8.3.0/../../../../xtensa-buildroot-linux-uclibc/bin/ld: ucm/.libs/libucm.a(main.o): in function `snd_use_case_set':
main.c:(.text+0x185c): undefined reference to `snd_mixer_selem_id_parse'

Fixes: http://autobuild.buildroot.org/results/4d91c9f82a2a61c50c457a851073b85cc09ea345
Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>