strcmp(3) needs string.h, posix test suite should know that.

Index: t0.c
--- t0.c.orig
+++ t0.c
@@ -40,6 +40,7 @@
 #include <pthread.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include <unistd.h>
 #include <signal.h>