samples/landlock: Fix building on musl libc

Building with make allyesconfig on musl results in the following

In file included from samples/landlock/sandboxer.c:22:
/usr/include/sys/prctl.h:88:8: error: redefinition of 'struct prctl_mm_map'
   88 | struct prctl_mm_map {
      |        ^~~~~~~~~~~~
In file included from samples/landlock/sandboxer.c:16:
usr/include/linux/prctl.h:134:8: note: originally defined here
  134 | struct prctl_mm_map {
      |        ^~~~~~~~~~~~

This is mainly due to difference in the sys/prctl.h between glibc and
musl. The struct prctl_mm_map is defined in sys/prctl.h in musl.

Signed-off-by: Brahmajit Das <listout@listout.xyz>
[mic: Move down the if/include/endif block]
Link: https://lore.kernel.org/r/20250630203248.16273-1-listout@listout.xyz
Signed-off-by: Mickaël Salaün <mic@digikod.net>

samples/landlock/sandboxer.c

@@ -13,7 +13,6 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <linux/landlock.h>
-#include <linux/prctl.h>
 #include <linux/socket.h>
 #include <stddef.h>
 #include <stdio.h>
@@ -25,6 +24,10 @@
 #include <unistd.h>
 #include <stdbool.h>
 
+#if defined(__GLIBC__)
+#include <linux/prctl.h>
+#endif
+
 #ifndef landlock_create_ruleset
 static inline int
 landlock_create_ruleset(const struct landlock_ruleset_attr *const attr,