nsresourced: don't redefine of bpf_rdonly_cast

bpf_rdonly_cast() was introduced in libbpf commit 688879f together with
the definition of a bpf_core_cast macro. So use that one to avoid
defining a prototype for bpf_rdonly_cast;

src/nsresourced/bpf/userns_restrict/userns-restrict.bpf.c

@@ -20,7 +20,12 @@
 #include <bpf/bpf_core_read.h>
 #include <errno.h>
 
+#ifndef bpf_core_cast
+/* bpf_rdonly_cast() was introduced in libbpf commit 688879f together with
+ * the definition of a bpf_core_cast macro. So use that one to avoid
+ * defining a prototype for bpf_rdonly_cast */
 void *bpf_rdonly_cast(void *, __u32) __ksym;
+#endif
 
 /* BPF module that implements an allowlist of mounts (identified by mount ID) for user namespaces (identified
  * by their inode number in nsfs) that restricts creation of inodes (which would inherit the callers UID/GID)