amend change to XawListChange()

My previous commit in July 2019

  build-fixes when _CONST_X_STRING is defined

modified the interface of XawListChange() to use "_Xconst char *"
to fix builds when the Xt symbol _CONST_X_STRING is defined (which
makes the String typedef "const").

While this fixed building Xaw, clients which use Xaw were impacted.
Changing that function to use String resolves both problems.

(report by Karl Berry).

Signed-off-by: Thomas E. Dickey <dickey@invisible-island.net>