Mark QtPrivate::ApplyReturnType constructor as explicit.

Else, the operator,(T, ApplyReturnType<void>) is sometimes chosen if a
pointer is passed, and that is breaking some decltype expressions.
(such as the one in ComputeFunctorArgumentCount in the next patch)

Change-Id: Ic203bbb1a8f5abbebb3b11786454807aa20be5fd
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qobjectdefs_impl.h

@@ -93,7 +93,7 @@ namespace QtPrivate {
     template <typename T>
     struct ApplyReturnValue {
         void *data;
-        ApplyReturnValue(void *data_) : data(data_) {}
+        explicit ApplyReturnValue(void *data_) : data(data_) {}
     };
     template<typename T, typename U>
     void operator,(const T &value, const ApplyReturnValue<U> &container) {