Add missing include of string.h.
https://bugzilla.redhat.com/show_bug.cgi?id=2159947

https://fedoraproject.org/wiki/Toolchain/PortingToModernC

diff --git a/hpwd.c b/hpwd.c
index cd3b100..84c34cf 100644
--- a/hpwd.c
+++ b/hpwd.c
@@ -24,6 +24,7 @@
 # endif
 
 # include <stdio.h>
+# include <string.h>
 
 # include "hfs.h"
 # include "hcwd.h"