Visual Studio Addin: IntermediateDirectory Fix.

Visual Studio 2012 and Visual Studio 2013 (and I suppose also VS2010)
warns during compile that the intermediate directory does not end with
a trailing slash in projects imported from .pro files.
During compilation of the project, Visual Studio adds the missing trailing
slash on its own but throws a warning.
This fix adds the missing trailing slash since it is hardcoded in ProjectImporter.cs

Task-number: QTVSADDINBUG-356

Change-Id: Ic1696abe375bc50798447668eeae694613e1260d
Reviewed-by: Torben Dannhauer <torben@dannhauer.info>
Reviewed-by: Ismo Haataja <ismo.haataja@digia.com>