Ignore warning for instantiation after specialization.

Bug: http://b/31532493

The false template specialization is indeed a duplicate, so we can
remove it, but the ART team would prefer to keep this explicit.

art/runtime/base/arena_allocator.cc:147:16: error: explicit
instantiation of 'ArenaAllocatorStatsImpl<false>' that occurs after an
explicit specialization has no effect
[-Werror,-Winstantiation-after-specialization]
template class ArenaAllocatorStatsImpl<kArenaAllocatorCountAllocations>;
               ^
art/runtime/base/arena_allocator.h:105:7: note: previous
template specialization is here
class ArenaAllocatorStatsImpl<false> {
      ^

Test: Builds correctly with latest toolchain.
Change-Id: Ifbcb3b98ff4451b9860a6c6c6b085745f89c0a64