cmake: include what you use

Add a missing include.  This is important in the case HandleLLVMOptions is
included prior to the missing CheckCXXSourceCompiles or CheckCXXCompilerFlag
which includes CheckCXXSourceCompiles.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@262949 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed