gitignore: ignore clangd .cache directory

gitignore: ignore clangd .cache directory

In at least some versions of clangd, including version 15 in Ubuntu
23.04, a directory, .cache, is written in the root of the repository
with index information about the files in the repository.  Since clangd
is the most common language server protocol (LSP) implementation for C,
and we already support it using the GENERATE_COMPILATION_DATABASE flags
to make it functional, it's likely many users are using or will want to
use it.

As a result, ignore the ".cache" directory to help avoid users
accidentally committing the data.

Signed-off-by: brian m. carlson <bk2204@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>