Change the .gitignore to ignore test-mktemp which is built from
test-mktemp.c. Arnout Engelen added this in 6cf6bb3 (Improve error
messages when temporary file creation fails, 2010-12-18) but forgot
to add a corresponding entry to .gitignore.
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>