Fix 'statement with no effect' warning

Because the secpolicy_* macros are all currently defined to (0).
And because the caller of this function does not check the return
code.  The compiler complains that this statement has no effect
which is correct and OK.  To suppress the warning explictly cast
the result to (void).

module/zfs/zfs_vnops.c

@@ -2274,7 +2274,7 @@ zfs_setattr(struct inode *ip, struct iattr *attr, int flags, cred_t *cr)
 				/*
 				 * Remove setuid/setgid for non-privileged users
 				 */
-				secpolicy_setid_clear(attr, cr);
+				(void) secpolicy_setid_clear(attr, cr);
 				trim_mask = (mask & (ATTR_UID|ATTR_GID));
 			} else {
 				need_policy =  TRUE;