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

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>