New parentheses warnings after HarfBuzz 7.2.0 update

At least GCC 6 fail the build with warnings-as-errors here:

```
../src/java.desktop/share/native/libharfbuzz/hb-subset-plan.cc: In function 'void _collect_layout_variation_indices(hb_subset_plan_t*)':
../src/java.desktop/share/native/libharfbuzz/hb-subset-plan.cc:389:83: error: suggest parentheses around assignment used as truth value [-Werror=parentheses]
     if (unlikely (!plan->check_success (font = _get_hb_font_with_variations (plan)))) {
                                                                                   ^
../src/java.desktop/share/native/libharfbuzz/hb.hh:259:46: note: in definition of macro 'unlikely'
 #define unlikely(expr) (__builtin_expect (!!(expr), 0))
                                              ^~~~
```

This is caused by HarfBuzz update to 7.2.0, JDK-8307301