AthExSYCL Debug Build Fix, master branch (2020.05.19.)

This is to fix the build of AthExSYCL in "Debug" builds.

As it turns out, the triSYCL headers don't work when using GCC's undefined behaviour sanitiser. Leading to last night's build failure.

http://atlas-computing.web.cern.ch/atlas-computing/links/distDirectory/gitwww/GITWebArea/nightlies/master/2020-05-18T2148/Athena/x86_64-centos7-gcc8-dbg/Athena/Control.AthenaExamples.AthExSYCL.log.html

So I turned it off for this package...

One could very well argue that atlas_setup_sycl_target(...) should be taught to do this behind the scenes. Which may very well happen in the next update of atlas/atlasexternals. But for now I wanted to provide a hot-fix for the build issue.