[binutils-gdb] Fix up wchar_iterator comment

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT b24b0d6c3be6b95d4b5e8da901e1ef315bd458b4 ***

Author: Tom Tromey <tom@tromey.com>
Branch: master
Commit: b24b0d6c3be6b95d4b5e8da901e1ef315bd458b4

Fix up wchar_iterator comment

This fixes up a comment in charset.h that has been obsolete for a
while.

gdb/ChangeLog
2017-04-12  Tom Tromey  <tom@tromey.com>

	* charset.h (wchar_iterator): Fix comment.