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: NEric Blake <eblake@redhat.com>

src/rpc/virnetserverclient.c

 /*
  * virnetserverclient.c: generic network RPC server client
  *
- * Copyright (C) 2006-2011 Red Hat, Inc.
+ * Copyright (C) 2006-2013 Red Hat, Inc.
  * Copyright (C) 2006 Daniel P. Berrange
  *
  * This library is free software; you can redistribute it and/or
@@ -23,6 +23,7 @@
 
 #include <config.h>
 
+#include "internal.h"
 #if WITH_SASL
 # include <sasl/sasl.h>
 #endif