[winlocks] Include stdlib.h to avoid crash in Windows

Due to the missing include, the compiler makes assumptions and leads to
a crash in ssh_mutex_lock() during runtime.

Signed-off-by: Chris Townsend <christopher.townsend@canonical.com>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>

src/threads/winlocks.c

@@ -26,6 +26,7 @@
 #include <windows.h>
 #include <winbase.h>
 #include <errno.h>
+#include <stdlib.h>
 
 static int ssh_winlock_mutex_init (void **priv)
 {