scripts/check-obsolete-constructs.py: Process all headers as UTF-8.

A few of our installed headers contain UTF-8 in comments.
check-obsolete-constructs opened files without explicitly specifying
their encoding, so it would barf on these headers if “make check” was
run in a non-UTF-8 locale.

	* scripts/check-obsolete-constructs.py (HeaderChecker.check):
	Specify encoding="utf-8" when opening headers to check.