[cmake] Fix libomptarget/test/CMakeLists.txt

Summary:
Should be variable name instead of variable reference. If the variable is
somehow unset, it messes up the if condition expression and causes a CMake
error.

Reviewers: jlpeyton, AndreyChurbanov, Hahnfeld

Reviewed By: Hahnfeld

Subscribers: mgorny, llvm-commits, openmp-commits

Differential Revision: https://reviews.llvm.org/D47221

llvm-svn: 337133