add string.h to logging.h

Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
diff --git a/include/logging.h b/include/logging.h
index 7b5f901..eef7dff 100644
--- a/include/logging.h
+++ b/include/logging.h
@@ -33,6 +33,7 @@
 #ifndef _LOGGING_H
 #define _LOGGING_H
 
+#include <string.h>
 #include <unistd.h>
 #include <linux/futex.h>