Move the declaration of smemclr() out of putty.h into misc.h, because

one of its uses (in sshaes.c) wasn't picking up the former. Thanks to
Ubuntu's gcc for spotting that.

[originally from svn r9590]

misc.h

@@ -50,6 +50,8 @@ void bufchain_fetch(bufchain *ch, void *data, int len);
 
 struct tm ltime(void);
 
+void smemclr(void *b, size_t len);
+
 /*
  * Debugging functions.
  *