[iter] Make operator bool explicit

We would need to write:

  for (Iter<T> it (...); bool (t); t++)

instead of:

  for (Iter<T> it (...); t; t++)

But I think it's an improvement in code readability in the long term.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1127

src/hb-iter-private.hh

@@ -72,7 +72,7 @@ struct Iter<T *>
     array (array_), length (length_) {}
 
   /* Emptiness. */
-  inline operator bool (void) const { return bool (length); }
+  explicit inline operator bool (void) const { return bool (length); }
 
   /* Current item. */
   inline T &operator * (void)