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.

utils/x11_parse_ip.c

@@ -6,6 +6,7 @@
 #include <stdio.h>
 
 #include "putty.h"
+#include "ssh.h"
 
 bool x11_parse_ip(const char *addr_string, unsigned long *ip)
 {