gdb: make so_list::lm_info a unique_ptr

Make it a unique_ptr, so it gets automatically deleted when the so_list
is deleted.

Change-Id: Ib62d60ae2a80656239860b80e4359121c93da13d
Approved-By: Pedro Alves <pedro@palves.net>
Reviewed-By: Reviewed-By: Lancelot Six <lancelot.six@amd.com>