Move Windows definition of CP_UTF8 into windows subdir.

I've only just noticed that the definition of CP_UTF8 as 65001 (the
Windows code page number for UTF-8) is in the main putty.h, under an
ifdef that checks whether the per-platform header file had already
defined it to something else. That's a silly way to do things! Better
that the Windows-specific definition goes _in_ the Windows platform
header, and putty.h contains no fallback. That way, anyone writing a
third separate platform directory will get an error reminding them
that they have to provide the right definition for their platform,
instead of finding out later via a runtime failure.

windows/platform.h

@@ -194,6 +194,7 @@ void centre_window(HWND hwnd);
 
 #define DEFAULT_CODEPAGE CP_ACP
 #define USES_VTLINE_HACK
+#define CP_UTF8 65001
 
 #ifndef NO_GSSAPI
 /*