cmake: remove base_name for dep file

With CMake 3.20 relative path inside DEPFILEs are treated relative to
CMAKE_CURRENT_BINARY_DIR and are transformed by CMake in its internal
dep file.

Therefore Zephyr build system must no longer add `base_name` to the
`-MT` argument for the preprocessor.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>