xsm: fix printf format string for strlen result

strlen returns size_t:

policydb.c: In function ‘policydb_read’:
policydb.c:1779: error: format ‘%lu’ expects type ‘long unsigned int’, but argument 3 has type ‘size_t’

This is probably benign on 64-bit x86 but was found by Dharshini on 32-bit Xen
4.2.x. I expect it affects ARM too.

Reported-by: Dharshini Tharmaraj <dharshinitharmaraj@gmail.com>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>

xen/xsm/flask/ss/policydb.c

@@ -1737,7 +1737,7 @@ int policydb_read(struct policydb *p, void *fp)
     if ( len != strlen(POLICYDB_STRING) )
     {
         printk(KERN_ERR "Flask:  policydb string length %d does not "
-               "match expected length %lu\n",
+               "match expected length %zu\n",
                len, strlen(POLICYDB_STRING));
         goto bad;
     }