compat/terminal: mark parameter of git_terminal_prompt() UNUSED

compat/terminal: mark parameter of git_terminal_prompt() UNUSED

If neither HAVE_DEV_TTY nor GIT_WINDOWS_NATIVE is set, the fallback
code calls the system getpass(). This unfortunately ignores the "echo"
boolean parameter, as we have no way to implement that functionality.
But we still have to keep the unused parameter, since our interface
has to match the other implementations.

Co-authored-by: Jeff King <peff@peff.net>
Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>