pm: cpupower: gitignore: Add compile_commands.json

A compile_commands.json file is used by the LSP in tools like VSCode and
Neovim to look up function and type information. The file is specific to
the state of the current system; add it to the gitignore.

Note: the kernel root's gitignore has a similar entry:

/compile_commands.json

I am not sure why they use '/' for a file as it is used for directories.

Signed-off-by: "John B. Wyatt IV" <jwyatt@redhat.com>
Signed-off-by: "John B. Wyatt IV" <sageofredondo@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>