regressions/ck_ring: Add missing header file.

regressions/ck_ring/validate/ck_ring_mpmc.c

@@ -27,6 +27,7 @@
 #include <assert.h>
 #include <stdlib.h>
 #include <stdio.h>
+#include <string.h>
 #include <pthread.h>
 
 #include <ck_ring.h>