clone: Make the 'junk_mode' symbol a file static

Sparse issues an "'junk_mode' not declared. Should it be static?"
warning. In order to suppress the warning, since this symbol does
not need more than file visibility, we simply add the static
modifier to its declaration.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>