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

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: 6955d9512d0e ("ASoC: SOF: Introduce IPC SOF client support")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Peter Ujfalusi <Peter Ujfalusi <peter.ujfalusi@linux.intel.com>>
Link: https://lore.kernel.org/r/20220214071330.22151-1-peter.ujfalusi@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>