gdb: make so_list::sections not a pointer

Make the field a vector directly, instead of a pointer to a vector.
This was needed when so_list had to be a trivial type, which is not the
case anymore.

Change-Id: I79a8378ce0d0d1e2206ca08a273ebf332cb3ba14
Approved-By: Pedro Alves <pedro@palves.net>
Reviewed-By: Reviewed-By: Lancelot Six <lancelot.six@amd.com>