codeowners: Fix CODEOWNERS for $ZEPHYR_BASE/CMakeLists.txt

CMakeLists.txt @nashif @SebastianBoe

was supposed to reference only the root file, but was referencing all
files. This commit patches CODEOWNERS to behave as intended.

See https://help.github.com/articles/about-codeowners/



Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>