landlock: fix building without landlock.h

landlock.h may not be available on the system (such as with older
versions of Linux API headers), so only try to include it if
`HAVE_LANDLOCK` is defined.

This fixes the following error from `build_debian_package` (which uses
`debian:buster`) on GitLab CI[1]:

    $ ./mkdeb.sh --enable-fatal-warnings
    [...]
    gcc [...] -c ../../src/firejail/landlock.c -o ../../src/firejail/landlock.o
    ../../src/firejail/landlock.c:22:10: fatal error: linux/landlock.h: No such file or directory
     #include <linux/landlock.h>
              ^~~~~~~~~~~~~~~~~~
    compilation terminated.

This amends commit a05ae97 ("landlock: amend empty functions and
comments", 2024-04-08) / PR #6305.

Relates to #6078.

[1] https://gitlab.com/Firejail/firejail_ci/-/jobs/6743161059

src/firejail/landlock.c

@@ -19,7 +19,6 @@
 */
 
 #include "firejail.h"
-#include <linux/landlock.h>
 #include <sys/prctl.h>
 #include <sys/syscall.h>
 #include <sys/types.h>
@@ -28,6 +27,8 @@
 
 #ifdef HAVE_LANDLOCK
 
+#include <linux/landlock.h>
+
 static int ll_ruleset_fd = -1;
 static int ll_abi = -1;