Disable erroneous old-style-cast warning

[kevingranade]

Summary

SUMMARY: None

Purpose of change

The Clang 6 CMAKE build target still reports a warning when this signal handler is invoked, see https://travis-ci.org/CleverRaven/Cataclysm-DDA/jobs/631788126

Describe the solution

Add pragmas to tell both GCC and clang to ignore this warning just at this location.

Describe alternatives you've considered

If this doesn't work we could disable this warning entirely just for this build target as it is the only place it is happening.

Testing

Using PR to test that I haven't broken something extremely basic, if that works the only simple way to verify is to push to master and check that the failing target starts succeeding.

[kevingranade]

I'm going to go ahead and push this to find out if it resolves the problem.