Add missing include guard

[joergbrech]

Description

How Has This Been Tested?

Screenshots, that help to understand the changes(if applicable):

Checklist:

Task	Finished	Reviewer Approved
At least one test for the new functionality was added.	yes does not apply	OK
New classes have been added to the Python interface.	yes does not apply	OK
The code is properly documented with doxygen docstrings	yes does not apply	OK
Changes are documented at the top of ChangeLog.md	yes does not apply	OK

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.48%. Comparing base (23808d7) to head (c29f99c).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1050   +/-   ##
=======================================
  Coverage   69.48%   69.48%           
=======================================
  Files         301      301           
  Lines       26894    26894           
=======================================
  Hits        18688    18688           
  Misses       8206     8206           

Flag	Coverage Δ
unittests	69.48% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/wing/CTiglWingSegmentList.h	100.00% <ø> (ø)