[aat] Make bots happy

Don't know why this is not caught by my compiler.  Looks like clang catches it
while gcc doesn't.

In file included from ../src/hb-aat-layout-morx-table.hh:31:
../src/hb-aat-layout-common-private.hh:523:30: error: no member named 'static_size' in 'AAT::ContextualSubtable<AAT::MorxTypes>::EntryData'
  DEFINE_SIZE_STATIC (4 + T::static_size);
                            ~~~^