landlock_common.h: Fix guarder + rename

Without definition the guarder does not work. Fix it and while at it
append '__' to the name (common LTP approach to avoid theoretical clash
with random header guarder).

Fixes: 4e17e90 ("Add landlock01 test")
Signed-off-by: Petr Vorel <pvorel@suse.cz>

testcases/kernel/syscalls/landlock/landlock_common.h

@@ -3,7 +3,8 @@
  * Copyright (C) 2024 SUSE LLC Andrea Cervesato <andrea.cervesato@suse.com>
  */
 
-#ifndef LANDLOCK_COMMON_H
+#ifndef LANDLOCK_COMMON_H__
+#define LANDLOCK_COMMON_H__
 
 #include "tst_test.h"
 #include "lapi/prctl.h"
@@ -71,4 +72,4 @@ static inline void apply_landlock_layer(
 	SAFE_CLOSE(ruleset_fd);
 }
 
-#endif
+#endif /* LANDLOCK_COMMON_H__ */