MDEV-25887 "Got notification message from PID xxxx, but reception only...

MDEV-25887 "Got notification message from PID xxxx, but reception only permitted for main PID yyyy" in systemd during SST

server has systemd support and calls sd_notify() to communicate
the status to systemd.

mariabackup links the whole server in, but it should not notify
systemd, because it's not started or managed by systemd.

extra/mariabackup/xtrabackup.cc

@@ -113,6 +113,12 @@ Street, Fifth Floor, Boston, MA 02110-1335 USA
 
 #define MB_CORRUPTED_PAGES_FILE "innodb_corrupted_pages"
 
+// disable server's systemd notification code
+extern "C" {
+int sd_notify() { return 0; }
+int sd_notifyf() { return 0; }
+}
+
 int sys_var_init();
 
 /* === xtrabackup specific options === */