Fix class vs struct mismatch

In file included from qtbase/include/QtCore/5.12.0/QtCore/private/qjni_p.h:1:
qtbase/include/QtCore/5.12.0/QtCore/private/../../../../../src/corelib/kernel/qjni_p.h:68:1: warning:
      'QJNIEnvironmentPrivate' defined as a class here but previously declared as a struct [-Wmismatched-tags]
class Q_CORE_EXPORT QJNIEnvironmentPrivate
^
jni/qandroidjnienvironment.h:50:1: note: did you mean class here?
struct QJNIEnvironmentPrivate;
^~~~~~
class

Change-Id: I0df1a4350c50ca920ad7f13978a62913a9702a15
Reviewed-by: Volker Krause <volker.krause@kdab.com>
Reviewed-by: Albert Astals Cid <albert.astals.cid@kdab.com>
Reviewed-by: BogDan Vatra <bogdan@kdab.com>

src/androidextras/jni/qandroidjnienvironment.h

@@ -47,7 +47,7 @@
 
 QT_BEGIN_NAMESPACE
 
-struct QJNIEnvironmentPrivate;
+class QJNIEnvironmentPrivate;
 
 class Q_ANDROIDEXTRAS_EXPORT QAndroidJniEnvironment
 {