gitignore: add clangd index files

https://clangd.llvm.org/design/indexing#backgroundindex

Signed-off-by: Tomi Fontanilles <129057597+tomi-font@users.noreply.github.com>
diff --git a/.gitignore b/.gitignore
index 185bd7a..4f29d5b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -63,5 +63,7 @@
 /cscope*.out
 /tags
 
-# Clangd compilation database
+# clangd compilation database
 compile_commands.json
+# clangd index files
+/.cache/clangd/index/