Also fix the unused variable warning that ICC found with Clang

The ICC fix happened on e02cb2b. This
now fixes for Clang 3.4:

cached-powers.cc:134:18: warning: unused variable 'kCachedPowersLength' [-Wunused-const-variable]

Change-Id: I876d9d5cf43fb7eb76117d6fdc37265295c360df
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>

src/qml/qml.pro

@@ -19,7 +19,9 @@ exists("qqml_enable_gcov") {
 QMAKE_DOCS = $$PWD/doc/qtqml.qdocconf
 
 # 2415: variable "xx" of static storage duration was declared but never referenced
+# unused variable 'xx' [-Werror,-Wunused-const-variable]
 intel_icc: WERROR += -ww2415
+clang: WERROR += -Wno-error=unused-const-variable
 
 load(qt_module)