target-ppc: Cast ssize_t to size_t before printing with %zx

The mingw32 compiler complains about trying to print variables of type
ssize_t with the %z format string specifier. Since we're printing it
as unsigned hex anyway, cast to size_t to silence the warning.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NAlexander Graf <agraf@suse.de>

hw/ppc/spapr.c

@@ -1482,7 +1482,7 @@ static void ppc_spapr_init(MachineState *machine)
     }
     if (spapr->rtas_size > RTAS_MAX_SIZE) {
         hw_error("RTAS too big ! 0x%zx bytes (max is 0x%x)\n",
-                 spapr->rtas_size, RTAS_MAX_SIZE);
+                 (size_t)spapr->rtas_size, RTAS_MAX_SIZE);
         exit(1);
     }
     g_free(filename);