Add .clangd configuration file that disables the unused include check

The clangd include checker is rather broken and is littered with false
positives in our codebase, so let's disable the feature until it improves
a bit.

.clangd

@@ -0,0 +1,4 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+Diagnostics:
+  UnusedIncludes: None