[coretext] Fix unused-variable error

Fixes https://github.com/harfbuzz/harfbuzz/issues/1659

src/hb-coretext.cc

diff --git a/src/hb-coretext.cc b/src/hb-coretext.cc
index 23f1c84..4923561 100644
--- a/src/hb-coretext.cc
+++ b/src/hb-coretext.cc
@@ -1069,7 +1069,7 @@
     if (false)
     {
       /* Make sure all runs had the expected direction. */
-      bool backward = HB_DIRECTION_IS_BACKWARD (buffer->props.direction);
+      HB_UNUSED bool backward = HB_DIRECTION_IS_BACKWARD (buffer->props.direction);
       assert (bool (status_and & kCTRunStatusRightToLeft) == backward);
       assert (bool (status_or  & kCTRunStatusRightToLeft) == backward);
     }