Ignore compile_commands.json file (#6472)

Summary:
Both clangd and cquery-language-server requires a compile_commands.json file to
index the project. This file can be ignored by git.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6472

Differential Revision: D20194899

Pulled By: riversand963

fbshipit-source-id: ea1587f2e5d10b7591147073b61efe262a1cf747

.gitignore

@@ -82,3 +82,5 @@ fbcode/
 fbcode
 buckifier/*.pyc
 buckifier/__pycache__
+
+compile_commands.json