Correct return type of QXmlQueryPrivate::staticContext()

As reported by UBSan:

  qtbase/src/corelib/tools/qshareddata.h:167:13: runtime error: downcast of address 0x2ac9a801a410 which does not point to an object of type 'GenericStaticContext'
  0x2ac9a801a410: note: object is of type 'QPatternist::StaticFocusContext'
   00 00 00 00  28 95 b7 9d c9 2a 00 00  01 00 00 00 c9 2a 00 00  f0 a1 01 a8 c9 2a 00 00  70 fa 0b 02
                ^~~~~~~~~~~~~~~~~~~~~~~
                vptr for 'QPatternist::StaticFocusContext'

I had no idea how the QExplicitlySharedDataPointer conversion
could ever compile, until I saw that this module defines

   QT_ENABLE_QEXPLICITLYSHAREDDATAPOINTER_STATICCAST

which is, of course, utterly broken.

Change-Id: Ie392ba74438b6c75fde9fabe09f9b0e655489cd9
Reviewed-by: Tobias Koenig <tobias.koenig@kdab.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@qt.io>

src/xmlpatterns/api/qxmlquery_p.h

@@ -122,7 +122,7 @@ public:
         return m_variableLoader;
     }
 
-    inline QPatternist::GenericStaticContext::Ptr staticContext()
+    inline QPatternist::StaticContext::Ptr staticContext()
     {
         if(m_staticContext && m_expr)
             return m_staticContext;