Fix build bug in ppc-linux-nat.c

Fix build bug in ppc-linux-nat.c

The buildbot pointed out that my value refactoring series introduced a
bug in ppc-linux-nat.c:

../../binutils-gdb/gdb/ppc-linux-nat.c: In member function ‘int ppc_linux_nat_target::num_memory_accesses(const std::vector<gdb::ref_ptr<value, value_ref_policy> >&)’:
../../binutils-gdb/gdb/ppc-linux-nat.c:2458:44: error: expected unqualified-id before ‘->’ token
 2458 |       if (VALUE_LVAL (v) == not_lval || v->->deprecated_modifiable () == 0)

I don't know how that happened, but I am checking in this patch which
I think should fix it.  It just removes the second "->".

I can't readily test this, so perhaps there's another bug lurking
after this one.