gen_compile_commands: fix missing 'sys' package

We need to import the 'sys' package since the script has called
sys.exit() method.

Fixes: 6ad7cbc0

 ("Makefile: Add clang-tidy and static analyzer support to makefile")
Signed-off-by: Kortan <kortanzh@gmail.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>