Add ASSERT_STATIC_EXPR macro

Unused right now.

src/hb-private.hh

diff --git a/src/hb-private.hh b/src/hb-private.hh
index ae52889..85561d2 100644
--- a/src/hb-private.hh
+++ b/src/hb-private.hh
@@ -78,6 +78,8 @@
 #define _ASSERT_STATIC0(_line, _cond) _ASSERT_STATIC1 (_line, (_cond))
 #define ASSERT_STATIC(_cond) _ASSERT_STATIC0 (__LINE__, (_cond))
 
+#define ASSERT_STATIC_EXPR(_cond) ((void) sizeof (char[(_cond) ? 1 : -1]))
+
 
 /* Lets assert int types.  Saves trouble down the road. */