gdb/dwarf: make init_cu_die_reader a method of cutu_reader

init_cu_die_reader is only used inside cutu_reader, to initialize fields
of cutu_reader, so make it a private method.

Change-Id: Iaa80d4dbb8d0fa35bcac18ee70e147276874cc1b
Approved-By: Tom Tromey <tom@tromey.com>