Include stdlib.h for free() prototype.

... which is used inside the CUSTOM_SYS_AUTH_GET_LASTLOGIN_MSG block.

sshlogin.c

@@ -50,6 +50,7 @@
 #include <fcntl.h>
 #include <stdarg.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 #include <time.h>
 #include <unistd.h>