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/utils/platform_get_x_display.c

@@ -4,6 +4,7 @@
  */
 
 #include "putty.h"
+#include "ssh.h"
 
 char *platform_get_x_display(void)
 {