remote.c: spell __attribute__ correctly

remote.c: spell __attribute__ correctly

We want to tell the compiler that error_buf() uses
printf()-style arguments via the __attribute__ mechanism,
but the original commit (3a429d0), forgot the trailing "__".
This happens to work with real GNUC-compatible compilers
like gcc and clang, but confuses our fallback macro in
git-compat-util.h, which only matches the official name (and
thus the build fails on compilers like Visual Studio).

Reported-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>