is_path_owned_by_current_uid(): mark "report" parameter as unused

is_path_owned_by_current_uid(): mark "report" parameter as unused

In the non-Windows version of this function, we never have any errors to
report, and thus the "report" parameter is unused. But we can't drop it,
because we have to maintain function call compatibility with the version
in compat/mingw.h, which does use this parameter.

Note that there's an extra level of indirection here; the common
function is actually is_path_owned_by_current_user, which is a macro
pointing to "by_current_uid" or "by_current_sid", depending on the
platform. So an alternative here is to eat the unused parameter in the
macro, since -Wunused-parameter doesn't complain about macros. But I
think the UNUSED() annotation is less obfuscated for somebody reading
the code later.

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