tools: add compile_commands to ignore file

For developing c++ source files, clangd generates an index file for
project via `./configure -C`. We can safely ignore the file from
tracking.

PR-URL: #41580
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

.gitignore

@@ -159,3 +159,6 @@ install_manifest.txt
 __pycache__
 .DS_Store
 *~
+
+# === Rules for C++ development ===
+compile_commands.json