kernel: fix build (#603)

kernel/sucompat.c

@@ -351,7 +351,13 @@ int ksu_handle_devpts(struct inode *inode)
 			return 0;
 
 	if (ksu_file_sid) {
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(5, 1, 0) ||						   \
+	defined(KSU_OPTIONAL_SELINUX_INODE)
 		struct inode_security_struct *sec = selinux_inode(inode);
+#else
+		struct inode_security_struct *sec =
+			(struct inode_security_struct *)inode->i_security;
+#endif
 		if (sec) {
 			sec->sid = ksu_file_sid;
 		}