Revert "Fix double definition of smemclr() on Windows."

This reverts commit 4982ed96fa.

(I failed to notice that Simon had already dealt with this in
188e2525c7 when I did a last-minute rebase.)

utils.c

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