Qt4ProjectManager: Skip files that are actually directories

Since the cumulative parsing now parses for loops, we get bogus data
in creator's own .pro files.

Task-number: QTCREATORBUG-10045
Change-Id: I627772f221cc5c928458788feb036cdd93b25a08
Reviewed-by: Robert Loehning <robert.loehning@digia.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>