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

Commit 4ce38a5 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>

src/ucm/main.c

@@ -2115,8 +2115,10 @@ int snd_use_case_parse_selem_id(snd_mixer_selem_id_t *dst,
 				const char *ucm_id,
 				const char *value)
 {
+#ifdef BUILD_MIXER
 	if (strcmp(ucm_id, "PlaybackMixerId") == 0 ||
 	    strcmp(ucm_id, "CaptureMixerId") == 0)
 		return snd_mixer_selem_id_parse(dst, value);
+#endif
 	return -EINVAL;
 }