GenericProject: Make dependency on CppTools optional

Especially in the light of the language server, the generic project is
currently the project one can use for language servers that require a
"project workspace".

Makes it possibly to run Qt Creator with
"-noload CppTools" if you still want to use generic
projects with some other language.

Change-Id: Ib9059289a2db4c44c0c1060a02fcdafacb885fbd
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/cpptools/cpptools.qbs

@@ -140,6 +140,7 @@ Project {
             "cppprojectpartchooser.h",
             "cppprojectupdater.cpp",
             "cppprojectupdater.h",
+            "cppprojectupdaterinterface.h",
             "cppqtstyleindenter.cpp",
             "cppqtstyleindenter.h",
             "cpprefactoringchanges.cpp",