CMake build: Build fix for automoc compilation failure

Compiler (MSVC, MinGW) got confused while trying to compile automoc's code
generated files for autotest/quick/quicktestparser.h

Autotest::TestTreeItem moved from Autotest::Internal::TestTreeItem, but
a few forward declarations remained in Autotest::Internal, which
resulted in errors like:

error C2555: overriding virtual function return type differs and is not
covariant

Side effect of commit 03b80025a9

Change-Id: I6e12526abc7e8f8a9562e7f2502e070e379117f4
Reviewed-by: hjk <hjk@qt.io>

src/plugins/autotest/boost/boosttestoutputreader.h

@@ -28,6 +28,9 @@
 #include "../testoutputreader.h"
 
 namespace Autotest {
+
+class TestTreeItem;
+
 namespace Internal {
 
 class BoostTestResult;

src/plugins/autotest/testframeworkmanager.h

@@ -37,6 +37,9 @@ QT_END_NAMESPACE
 namespace Core { class Id; }
 
 namespace Autotest {
+
+class TestTreeItem;
+
 namespace Internal {
 
 class TestRunner;
@@ -47,7 +50,6 @@ struct TestSettings;
 class IFrameworkSettings;
 class ITestParser;
 class ITestSettingsPage;
-class TestTreeItem;
 class TestTreeModel;
 
 class TestFrameworkManager