gdb/dwarf: make dwarf2_get_dwz_file a method of dwarf2_per_bfd

gdb/dwarf: make dwarf2_get_dwz_file a method of dwarf2_per_bfd

dwarf2_get_dwz_file looks more or less like a simple getter of
dwarf2_per_bfd::dwz_file, so make it into a method.

I typically avoid the `get_` prefix for getters, but that would conflict
with the field name here.

Change-Id: Idd0d5b1bd3813babf438b20aac514b19c77cfc18
Approved-By: Tom Tromey <tom@tromey.com>