Add tmpfile_003_pos to .gitignore

Earlier commit added a test that created tmpfile_003_pos,
but did not add it to the appropriate .gitignore.

ace1eae Add support for O_TMPFILE

Fix the omission.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes #5735