git-compat-util: move content inside ifdef/endif guards

git-compat-util: move content inside ifdef/endif guards

Commit 3f2e2297b9 (add an extra level of indirection to
main(), 2016-07-01) added a declaration to git-compat-util.h,
but it was accidentally placed after the final #endif that
guards against multiple inclusions.

This doesn't have any actual impact on the code, since it's
not incorrect to repeat a function declaration in C. But
it's a bad habit, and makes it more likely for somebody else
to make the same mistake. It also defeats gcc's optimization
to avoid opening header files whose contents are completely
guarded.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>