Fix compiler warning/error with clang 3.6

Clang warns about self assignment, so use the same mechanism as
Q_UNUSED for HB_UNUSED.

Change-Id: I0894c72fb0936074b15198053464f5bc4b8991d4
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>

src/3rdparty/harfbuzz/src/harfbuzz-impl.h

@@ -59,7 +59,7 @@ HB_BEGIN_HEADER
 #endif
 
 #ifndef HB_UNUSED
-# define HB_UNUSED(arg) ((arg) = (arg))
+# define HB_UNUSED(arg) ((void)(arg))
 #endif
 
 #define HB_LIKELY(cond) (cond)