C++: Custom directory list for Switch Header/Source

Some projects use separate directories for sources and headers.

An example tree:
*
|-- src
     |-- foo.cpp
|-- include
     |-- foo.h

Allow the user to specify directories for finding out-of-project related
header/source files, in addition to current directory

Task-number: QTCREATORBUG-8883
Change-Id: I57215c8f2feffcc246d0d161798290861bcfcdd4
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

src/plugins/cpptools/cpptools.qbs

@@ -118,6 +118,7 @@ QtcPlugin {
         files: [
             "cppcodegen_test.cpp",
             "cppcompletion_test.cpp",
+            "cppheadersource_test.cpp",
             "cppmodelmanager_test.cpp",
             "modelmanagertesthelper.cpp", "modelmanagertesthelper.h",
             "cpppointerdeclarationformatter_test.cpp",