.gitignore: Add .clangd as a "Development friendly file"

If using clangd for development, your .clangd file is almost
certainly specific to your environment, and should be gitignored,
same as e.g. .vscode/

Change-Id: I3388d14f381aa9f68be9806652514a741fad49c9
Signed-off-by: Dodoid <git-noreply@dodoid.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/89680
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nicholas <nic.c3.14@gmail.com>

.gitignore

@@ -35,6 +35,7 @@ tags
 .cache
 compile_commands.json
 .vscode/
+.clangd
 
 # Cross-compile toolkits
 xgcc/