Move defn of PLATFORM_HAS_SMEMCLR into defs.h.

After I moved parts of misc.c into utils.c, we started getting two
versions of smemclr in the Windows builds, because utils.c didn't know
to omit its one, having not included the main putty.h.

But it was deliberate that utils.c didn't include putty.h, because I
wanted it (along with the rest of testcrypt in particular) to be
portable to unusual platforms without having to port the whole of the
code base.

So I've moved into the ubiquitous defs.h just the one decision about
whether we're on a platform that will supersede utils.c's definition
of smemclr.

(Also, in the process of moving it, I've removed the clause that
disabled the Windows smemclr in winelib mode, because it looks as if
the claim that winelib doesn't have SecureZeroMemory is now out of
date.)

windows/winstuff.h

@@ -101,14 +101,6 @@ struct FontSpec *fontspec_new(
 #define BOXRESULT (DLGWINDOWEXTRA + sizeof(LONG_PTR))
 #define DF_END 0x0001
 
-#ifdef __WINE__
-#define NO_SECUREZEROMEMORY            /* winelib doesn't have this */
-#endif
-
-#ifndef NO_SECUREZEROMEMORY
-#define PLATFORM_HAS_SMEMCLR /* inhibit cross-platform one in misc.c */
-#endif
-
 #ifndef __WINE__
 /* Up-to-date Windows headers warn that the unprefixed versions of
  * these names are deprecated. */