gdb: Rename structures within ctfread.c

Commit:

  commit 30d1f018
  Date:   Mon Oct 7 00:46:52 2019 +0000

      gdb: CTF support

Introduces some structures with names that are already in use within
GBB, this violates C++'s one-definition rule.  Specifically the
structures 'nextfield' and 'field_info' are now defined in
dwarf2read.c and ctfread.c.

This commit renames the new structures (in ctfread.c), adding a 'ctf_'
prefix.  Maybe we should consider renaming the DWARF versions too in
the future to avoid accidental conflicts.

gdb/ChangeLog:

	* ctfread.c (struct nextfield): Renamed to ...
	(struct ctf_nextfield): ... this.
	(struct field_info): Renamed to ...
	(strut ctf_field_info): ... this.
	(attach_fields_to_type): Update for renamed structures.
	(ctf_add_member_cb): Likewise.
	(ctf_add_enum_member_cb): Likewise.
	(process_struct_members): Likewise.
	(process_enum_type): Likewise.