QCryptographicHash: make constructor explicit

The copy constructor of QCH is disabled, so there's
no point in providing an implicit conversion from
the Algorithm enum anyway, so make the ctor explicit.

Change-Id: I4ea74ffb0963b4f49415da17778c3e6050454a6b
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/corelib/tools/qcryptographichash.h

@@ -65,7 +65,7 @@ public:
         Sha512
     };
 
-    QCryptographicHash(Algorithm method);
+    explicit QCryptographicHash(Algorithm method);
     ~QCryptographicHash();
 
     void reset();