client: Add a missing #include <string.h>

play.c needs <string.h> since commit f1fc0df.  My version of
<readline/readline.h> pulls it in, but at least Apple's (derived from
NetBSD's) doesn't, and we get warnings then.  Add the missing

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>

src/client/play.c

@@ -39,6 +39,7 @@
 #include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #ifdef _WIN32
 #include <process.h>
 #include <sys/socket.h>