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/window.c

@@ -14,6 +14,7 @@
 #define COMPILE_MULTIMON_STUBS
 
 #include "putty.h"
+#include "ssh.h"
 #include "terminal.h"
 #include "storage.h"
 #include "putty-rc.h"