Add some missing #includes.

These are all intended to ensure that the declarations of things in
header files are in scope where the same thing is subsequently
defined, to make it harder to define it in a way that doesn't match.
(For example, the new #include in winnet.c would have caught the
just-fixed mis-definition of platform_get_x11_unix_address.)

unix/uxplink.c

@@ -17,6 +17,7 @@
 
 #define PUTTY_DO_GLOBALS               /* actually _define_ globals */
 #include "putty.h"
+#include "ssh.h"
 #include "storage.h"
 #include "tree234.h"
 

unix/uxputty.c

@@ -13,6 +13,7 @@
 #define MAY_REFER_TO_GTK_IN_HEADERS
 
 #include "putty.h"
+#include "ssh.h"
 #include "storage.h"
 
 #include "gtkcompat.h"