ASoC: SOF: Makefile: Fix randconfig sof-client build when SND_SOC_SOF=y

Intel's kernel test robot found the following randconfig combination:
SND_SOC_SOF=y
SND_SOC_SOF_CLIENT=m

In this the sof-client object is not going to be built into the snd-sof.o
and we will have undefined references to the sof-client functions.

Fixes: 6955d951 ("ASoC: SOF: Introduce IPC SOF client support")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Peter Ujfalusi &lt;Peter Ujfalusi <peter.ujfalusi@linux.intel.com&gt;>
Link: https://lore.kernel.org/r/20220214071330.22151-1-peter.ujfalusi@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/sof/Makefile

@@ -2,7 +2,9 @@
 
 snd-sof-objs := core.o ops.o loader.o ipc.o pcm.o pm.o debug.o topology.o\
 		control.o trace.o iomem-utils.o sof-audio.o stream-ipc.o
-snd-sof-$(CONFIG_SND_SOC_SOF_CLIENT) += sof-client.o
+ifneq ($(CONFIG_SND_SOC_SOF_CLIENT),)
+snd-sof-objs += sof-client.o
+endif
 
 snd-sof-$(CONFIG_SND_SOC_SOF_COMPRESS) += compress.o