add .clangd to .gitignore

As with may other Git-ignored items - whenever we do not have this file
in Git, make it ignored to allow the developers to have therir own
(without being listed as untracked).

Author: MartinPulec

.gitignore

@@ -57,6 +57,7 @@ unittest/run_tests
 .cache
 .clang-format
 .clang-tidy
+.clangd
 .editorconfig
 .envrc
 compile_commands.json