[CMake][runtimes] Fix the variable name

This typo causes the llvm-lit path resolution to fail.

Differential Revision: https://reviews.llvm.org/D39811

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317742 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed