gdb: constify breakpoint::print_it parameter

gdb: constify breakpoint::print_it parameter

The print_it method itself is const.  In a subsequent patch, the
locations that come out of a const breakpoint will be const as well.  It
will therefore be needed to make the last_loc output parameter const as
well.  Make that change now to reduce the size of the following patches.

Change-Id: I7ed962950bc9582646e31e2e42beca2a1c9c5105
Reviewed-By: Andrew Burgess <aburgess@redhat.com>