cyrus-sasl: ensure size_t is defined for use by sasl.h

While making use of the cyrus-sasl library an external application
will most likely include sasl.h which will cause compilation failures
if steps haven't been taken to define size_t. This should not be the
responsibility of the application building against the sasl library
but rather handled already in sasl.h. Here we ensure sasl.h has the
proper includes to have size_t defined before it is used.

Signed-off-by: Mark Asselstine <mark.asselstine@windriver.com>
Signed-off-by: Joe MacDonald <joe@deserted.net>