[binutils-gdb] Use std::string in ppscm_make_pp_type_error_exception

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT 7eb1a66c26b57803bb4245f84cf15278297bb649 ***

Author: Tom Tromey <tom@tromey.com>
Branch: master
Commit: 7eb1a66c26b57803bb4245f84cf15278297bb649

Use std::string in ppscm_make_pp_type_error_exception

This changes ppscm_make_pp_type_error_exception to use std::string,
removing a cleanup.

gdb/ChangeLog
2018-07-17  Tom Tromey  <tom@tromey.com>

	* guile/scm-pretty-print.c (ppscm_make_pp_type_error_exception):
	Use string_printf.