build: work around broken sasl header

build: work around broken sasl header

Compilation on cygwin failed due to a bug in the sasl headers
present on that platform (libsasl2-devel 2.1.26):

In file included from rpc/virnetserverclient.c:27:0:
/usr/include/sasl/sasl.h:230:38: error: expected declaration specifiers or '...' before 'size_t'

Upstream is aware of their bug:
https://bugzilla.cyrusimap.org/show_bug.cgi?id=3759

* src/rpc/virnetserverclient.c (includes): Ensure size_t is
defined before using sasl.h.

Signed-off-by: Eric Blake <eblake@redhat.com>