error-notify: Explicitly include string.h for strcpy()

src/libcharon/plugins/error_notify/error_notify.c

@@ -19,6 +19,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <stddef.h>
+#include <string.h>
 #include <unistd.h>
 #include <sys/stat.h>
 #include <sys/socket.h>