Add a missing include to putty.h.

We define a macro in terms of INT_MAX, so we ought to include
<limits.h> to ensure INT_MAX is defined, rather than depending on
every call site to have remembered to do that themselves.

putty.h

@@ -2,6 +2,7 @@
 #define PUTTY_PUTTY_H
 
 #include <stddef.h>		       /* for wchar_t */
+#include <limits.h>                    /* for INT_MAX */
 
 /*
  * Global variables. Most modules declare these `extern', but