clang: include tooltips: fix range check

Summary:
The clang plugin would always show the include navigation tooltip
if the cursor was anywhere on the line, since it was never checking
whether the cursor actually contains the word range.

Reviewers: kfunk

Reviewed By: kfunk

Subscribers: kdevelop-devel

Tags: #kdevelop

Differential Revision: https://phabricator.kde.org/D14865