Fix "discards 'const' qualifier from pointer target type" warnings

The resources pointed to have been acquired by the backend.  It is safe
to cast away the 'const' qualifier when releasing these resources.

backend/epjitsu.c

@@ -4576,13 +4576,13 @@ destroy(struct scanner *s)
     teardown_buffers(s);
 
     if(s->sane.name){
-      free(s->sane.name);
+      free((void *) s->sane.name);
     }
     if(s->sane.vendor){
-      free(s->sane.vendor);
+      free((void *) s->sane.vendor);
     }
     if(s->sane.model){
-      free(s->sane.model);
+      free((void *) s->sane.model);
     }
   
     free(s);

backend/hp4200.c

@@ -2437,7 +2437,7 @@ sane_exit (void)
 	}
       if (device->dev.name)
 	{
-	  free (device->dev.name);
+	  free ((void *) device->dev.name);
 	}
       free (device);
     }

backend/rts8891.c

@@ -2233,11 +2233,11 @@ sane_close (SANE_Handle handle)
   if (session->dev->model->gamma != session->val[OPT_GAMMA_VECTOR_B].wa)
     free (session->val[OPT_GAMMA_VECTOR_B].wa);
   free (session->val[OPT_MODE].s);
-  free (session->opt[OPT_RESOLUTION].constraint.word_list);
+  free ((void *) session->opt[OPT_RESOLUTION].constraint.word_list);
   for (i = OPT_BUTTON_1; i <= OPT_BUTTON_11; i++)
     {
-      free (session->opt[i].name);
-      free (session->opt[i].title);
+      free ((void *) session->opt[i].name);
+      free ((void *) session->opt[i].title);
     }
 
   free (session);

backend/umax_pp.c

@@ -1001,9 +1001,9 @@ sane_exit (void)
   for (i = 0; i < num_devices; i++)
     {
       free (devlist[i].port);
-      free (devlist[i].sane.name);
-      free (devlist[i].sane.model);
-      free (devlist[i].sane.vendor);
+      free ((void *) devlist[i].sane.name);
+      free ((void *) devlist[i].sane.model);
+      free ((void *) devlist[i].sane.vendor);
     }
 
   if (devlist != NULL)