config.h: fix hdr-check warnings

config.h: fix hdr-check warnings

commit 8f7c7f5555 ("config.c: add repo_config_set_worktree_gently()",
2018-12-27) adds three function declarations that cause the hdr-check
make target to complain. The hdr-check complaint is caused by placing
the new declarations, which include 'struct repository *' parameters,
before the declaration of 'struct repository'. Move the struct
declaration to the top of the file.

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>