Explicitly set CMAKE_GENERATOR

CMake based builds for reference decoder follow the schema:
cmake <bla> && $(MAKE) -C build
This only works for 'Unix Makefile' generator. This breaks if the user
has set CMAKE_GENERATOR by default to something different, e.g. Ninja.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>

Makefile

@@ -8,6 +8,7 @@ FLUSTER+=--no-emoji
 else
 KERNEL_NAME=$(shell uname -s)
 endif
+CMAKE_GENERATOR=Unix Makefiles
 
 help:
 	@awk -F ':|##' '/^[^\t].+?:.*?##/ { printf "\033[36m%-30s\033[0m %s\n", $$1, $$NF }' $(MAKEFILE_LIST)