gdb/dwarf: use dwarf2_per_cu::ref_addr_size in one spot

I noticed this spot that could use dwarf2_per_cu::ref_addr_size, instead
of implementing the same logic.

Change-Id: I894ef1dcaa687c659e49bc1eb433e42818038cf2
Approved-By: Andrew Burgess <aburgess@redhat.com>
1 file changed