Make sure QMimeData is fully declared

The test deletes instances of QMimeData in the dropMimeData function.
The compiler warns about deleting objects of incomplete type if QMimeDate
is only forward declared.

Change-Id: I3423a7ea334180ff0b68efbecb3d3feeb0632239
Reviewed-by: David Skoland <david.skoland@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

tests/auto/corelib/itemmodels/qabstractitemmodel/tst_qabstractitemmodel.cpp

@@ -38,6 +38,8 @@
 // for testing QModelRoleDataSpan construction
 #include <QVarLengthArray>
 #include <QSignalSpy>
+#include <QMimeData>
+
 #include <array>
 #include <vector>
 #include <deque>