auth: Add missing include for explicit_bzero()

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>
(cherry picked from commit 4a113159b2)

src/auth.c

@@ -25,6 +25,7 @@
 #include "config.h"
 
 #include <stdio.h>
+#include <string.h>
 
 #ifndef _WIN32
 #include <netinet/in.h>