...since we can always provide the contents for such files.

This fixes the soft assert

    SOFT ASSERT: "document()" in file
	src/plugins/cpptools/cppsnapshotupdater.cpp, line 199

that occurs if you Follow Symbol on an include directive of the type
"ui_*.h" file that is not yet generated on disk.

Change-Id: I1e40db95a648a0570c6ccbcf5475691094097e18
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>