QHash: default the argument for the hash calculator helper

Just to be consistent with everything else.

Change-Id: I48ceb4bbc1cbf65b03caee77b7405cb585793248
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qhash.h

@@ -85,7 +85,7 @@ constexpr inline bool HasStdHashSpecializationWithoutSeed<T, std::enable_if_t<
 >> = true;
 
 template <typename T>
-size_t calculateHash(const T &t, size_t seed)
+size_t calculateHash(const T &t, size_t seed = 0)
 {
     if constexpr (HasQHashOverload<T>) {
         return qHash(t, seed);