ASoC: Makefile: Fix simultaneous build of KUNIT tests

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: Amadeusz Sławiński <amadeuszx.slawinski@linux.intel.com>
Reviewed-by: Cezary Rojewski <cezary.rojewski@intel.com>
Link: https://lore.kernel.org/r/20220720125115.1785426-1-amadeuszx.slawinski@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>