Config.uk: Add posix-tty as dependency to stdio

Config.uk: Add posix-tty as dependency to stdio

This change adds an explicit dependency on posix-tty and its STDIO
option for when the libstdio sub-lib of musl is enabled.
This makes musl's assumption that fds 0, 1, 2 be opened to something
meaningful an explicit configuration requirement on the kernel.

Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Razvan Deaconescu <razvand@unikraft.io>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
GitHub-Closes: #85