man: Ensure notify example includes <string.h>

This ensures that memcpy and strerror are defined.  This is especially
important with GCC 14 as implicit function declarations are now an error.

man/notify-selfcontained-example.c

@@ -15,6 +15,7 @@
 #include <stddef.h>
 #include <stdlib.h>
 #include <stdio.h>
+#include <string.h>
 #include <sys/socket.h>
 #include <sys/un.h>
 #include <time.h>