libseccomp: fix typo in DESCRIPTION

Fix typo in DESCRIPTION: and -> an.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 349e6304)
Signed-off-by: Steve Sakoman <steve@sakoman.com>