build: Update m4/iconv.m4.
* m4/iconv.m4: Update from gettext 0.20.1.
Cherry-picked from master commit:
1cd2aca03b8807c6f8e4929ace462bb606dcd53f
This includes fixes of file descriptor leaks.
- GnuPG-bug-id: T4504
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>