Fixed compilation error on RHEL 5.10.

Nesting the namespace "Internal" inside of other namespaces and using
sequential "using namespace" directives *or* relying on a "using
namespace" directive to resolve a method name that contained the
"Internal" namespace for resolution resulted in ambiguous resolution
errors when compiling with the RHEL5.10 stock compiler (GCC 4.1.2).  A
sample error is provided below:

Compilation error encountered was:
reference to 'Internal' is ambiguous
/usr/lib/gcc/x86_64-redhat-
candidates are: namespace Internal { }
qt-creator/src/libs/cplusplus/PPToken.h:106: error:
namespace CPlusPlus::Internal { }

Modifying the using namespace directives to reflect the nested nature of
the namespace names resolved the compilation errors.  For example:

using namespace CppEditor;
using namespace Internal;

produced errors.  However,

using namespace CppEditor;
using namespace CppEditor::Internal;

resolved the compilation errors by removing the ambiguity.

Change-Id: I6d5051e20acb2c147bd7ee1c6c6e1e3b2b4ff0f9
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

src/plugins/cppeditor/cppincludehierarchy.cpp

@@ -50,7 +50,7 @@
 #include <QVBoxLayout>
 
 using namespace CppEditor;
-using namespace Internal;
+using namespace CppEditor::Internal;
 using namespace Utils;
 
 namespace CppEditor {