libshout: fix typo

Fix error:
  package/libshout/libshout.mk:28: *** Package error: use LIBSHOUT_CONF_OPTS instead of LIBSHOUT_CONF_OPT. Please fix your .mk file.  Stop.

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>