Fix another compiler warning.

ppdc/ppdc-array.cxx

@@ -35,7 +35,7 @@ ppdcArray::ppdcArray(ppdcArray *a)
 
       memcpy(data, a->data, (size_t)count * sizeof(ppdcShared *));
 
-      for (int i = 0; i < count; i ++)
+      for (size_t i = 0; i < count; i ++)
         data[i]->retain();
     }
     else