.gitignore: ignore compile_commands.json globally

.gitignore: ignore compile_commands.json globally

compile_commands.json can be used with clangd to enable language server
protocol-based assistance. For kernel itself this can be built with
scripts/gen_compile_commands.py, but other projects (e.g., libbpf, or
BPF selftests) can benefit from their own compilation database file,
which can be generated successfully using external tools, like bear [0].

So, instead of adding compile_commands.json to .gitignore in respective
individual projects, let's just ignore it globally anywhere in Linux repo.

While at it, remove exactly such a local .gitignore rule under
tools/power/cpupower.

  [0] https://github.com/rizsotto/Bear

Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Suggested-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://lore.kernel.org/r/20250606214840.3165754-1-andrii@kernel.org
Signed-off-by: Nathan Chancellor <nathan@kernel.org>