gitignore: Add compile_commands.json

The purpose of this change is to make sure no one would accidentally
include their own workspace leftover into a commit.

compile_commands.json is a generated file which contains build commands.
The file is consumed by Language Servers such as clangd and ccls.

Sponsored by: The FreeBSD Foundation
Reviewed by: lwhsu
Differential Revision: https://reviews.freebsd.org/D26514