Use unique_xmalloc_ptr in solib-aix.c

This removes a cleanup from solib-aix.c via unique_xmalloc_ptr.

gdb/ChangeLog
2018-06-18  Tom Tromey  <tom@tromey.com>

	* solib-aix.c (solib_aix_get_section_offsets): Return
	unique_xmalloc_ptr.
	(solib_aix_solib_create_inferior_hook): Update.