Prevent clang from warning about unused variables

Fixes a compiler warning introduced by 79b975756a

Change-Id: Ie9bc677d061f0bb0530eae88715069042b2ab440
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>

src/corelib/io/qloggingcategory.cpp

@@ -115,6 +115,9 @@ QLoggingCategory::QLoggingCategory(const char *category)
       enabledWarning(true),
       enabledCritical(true)
 {
+    Q_UNUSED(d);
+    Q_UNUSED(placeholder);
+
     bool isDefaultCategory
             = (category == 0) || (strcmp(category, qtDefaultCategoryName) == 0);