Use std::string in reread_symbols

Use std::string in reread_symbols

This removes a cleanup from reread_symbols by using std::string.  This
fixes a memory leak, because this cleanup is ordinarily discarded, not
run.

Tested by the buildbot.

ChangeLog
2018-05-21  Tom Tromey  <tom@tromey.com>

	* symfile.c (reread_symbols): Use std::string for original_name.