glue.c: Remove multiple definitions when using Musl

glue.c: Remove multiple definitions when using Musl

Musl provides the `catclose`, `catopen` and `catgets` functions, so when
building `libcxx` with Musl the linking fails.

This commit only defines the needed functions when `LIBMUSL` is not selected.

Signed-off-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Maria Sfiraiala <maria.sfiraiala@gmail.com>
Reviewed-by: Razvan Deaconescu <razvand@unikraft.io>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #17