[BUILD] remove a warning in backend.c

include <string.h> to remove a warning on memset
diff --git a/src/backend.c b/src/backend.c
index 1813d34..4d8c613 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -15,6 +15,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <syslog.h>
+#include <string.h>
 
 #include <common/compat.h>
 #include <common/config.h>