Fix double definition of smemclr() on Windows.

PLATFORM_HAS_SMEMCLR from winstuff.h was available to misc.c via
putty.h, but the point of utils.c is not to pull in that stuff.

This is a quick bodge to unbreak the Windows build. It needs a better
answer for optionally overriding the platform-independent smemclr()
with a platform-specific implementation.

utils.c

@@ -850,7 +850,7 @@ void debug_memdump(const void *buf, int len, bool L)
 
 #endif				/* def DEBUG */
 
-#ifndef PLATFORM_HAS_SMEMCLR
+#ifndef _WINDOWS /* FIXME PLATFORM_HAS_SMEMCLR */
 /*
  * Securely wipe memory.
  *