promisor-remote.h: include missing header

promisor-remote.h: include missing header

When we ran `make hdr-check`, we got the following warning message:

	promisor-remote.h:21:46: warning: declaration of 'struct repository' will not be visible outside of this function [-Wvisibility]
	extern int promisor_remote_get_direct(struct repository *repo,
						     ^
	1 warning generated.

This was caused by a missing reference to `struct repository`, which is
defined in "repository.h".

Include this missing header to fix this warning.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>