[binutils-gdb] Remove cleanups from utils.c

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT 200aa7b154b25423eb72ceecade0fcd76edc4686 ***

Author: Tom Tromey <tom@tromey.com>
Branch: master
Commit: 200aa7b154b25423eb72ceecade0fcd76edc4686

Remove cleanups from utils.c

This removes a couple of cleanups from utils.c through the use of
std::string.

gdb/ChangeLog
2017-09-29  Tom Tromey  <tom@tromey.com>

	* utils.c (vfprintf_maybe_filtered): Use std::string.
	(vfprintf_unfiltered): Likewise.