Add some missing #includes.

My experimental build with clang-cl at -Wall did show up a few things
that are safe enough to fix right now. One was this list of missing
includes, which was causing a lot of -Wmissing-prototype warnings, and
is a real risk because it means the declarations in headers weren't
being type-checked against the actual function definitions.

Happily, no actual mismatches.

windows/plink.c

@@ -8,6 +8,7 @@
 #include <stdarg.h>
 
 #include "putty.h"
+#include "ssh.h"
 #include "storage.h"
 #include "tree234.h"
 #include "security-api.h"