Fix leak in solib-target.c:library_list_start_library

lm_info_target::name is nowadays std::string, so we're leaking the
result of xstrdup.

gdb/ChangeLog:
2019-01-10  Pedro Alves  <palves@redhat.com>

	* solib-target.c (library_list_start_library): Don't xstrdup name.