gitignore: add test-mktemp to ignore list

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>

.gitignore

@@ -170,6 +170,7 @@
 /test-index-version
 /test-line-buffer
 /test-match-trees
+/test-mktemp
 /test-obj-pool
 /test-parse-options
 /test-path-utils