builtin/fetch.c: Fix a sparse warning

Sparse issues an "'prepare_transport' was not declared. Should it
be static?" warning. In order to suppress the warning, since this
symbol only requires file scope, we simply add the static modifier
to it's declaration.

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