ASoC: Makefile: Fix simultaneous build of KUNIT tests

Using obj-$() := instead of obj-$() += leads to the latter assignment
overwriting earlier value. Fix this by using incremental assignment to
add additional objects to build.
Signed-off-by: NAmadeusz Sławiński <amadeuszx.slawinski@linux.intel.com>
Reviewed-by: NCezary Rojewski <cezary.rojewski@intel.com>
Link: https://lore.kernel.org/r/20220720125115.1785426-1-amadeuszx.slawinski@linux.intel.comSigned-off-by: NMark Brown <broonie@kernel.org>

sound/soc/Makefile

@@ -9,12 +9,12 @@ endif
 
 ifneq ($(CONFIG_SND_SOC_TOPOLOGY_KUNIT_TEST),)
 # snd-soc-test-objs := soc-topology-test.o
-obj-$(CONFIG_SND_SOC_TOPOLOGY_KUNIT_TEST) := soc-topology-test.o
+obj-$(CONFIG_SND_SOC_TOPOLOGY_KUNIT_TEST) += soc-topology-test.o
 endif
 
 ifneq ($(CONFIG_SND_SOC_UTILS_KUNIT_TEST),)
 # snd-soc-test-objs := soc-utils-test.o
-obj-$(CONFIG_SND_SOC_UTILS_KUNIT_TEST) := soc-utils-test.o
+obj-$(CONFIG_SND_SOC_UTILS_KUNIT_TEST) += soc-utils-test.o
 endif
 
 ifneq ($(CONFIG_SND_SOC_GENERIC_DMAENGINE_PCM),)