Include string.h where we use memset

This is needed on Solaris/Illumos/OpenIndiana/SunOS.

Presumably it's harmless elsewhere.

src/io.cpp

@@ -4,6 +4,7 @@
 #include <errno.h>
 #include <stddef.h>
 #include <stdio.h>
+#include <string.h>
 #include <unistd.h>
 #include <wchar.h>
 

src/iothread.cpp

@@ -4,6 +4,7 @@
 #include <pthread.h>
 #include <signal.h>
 #include <stdio.h>
+#include <string.h>
 #include <sys/select.h>
 #include <sys/time.h>
 #include <sys/types.h>