AddLLVM: squelch CMP0007 by not adding empty elements to list

When configuring various llvm projects that use AddLLVM.cmake, this warning is
emitted many times, flooding the screen:

Policy CMP0007 is not set: list command no longer ignores empty elements.

The fix is removing an extra semicolon.

Differential Revision: http://reviews.llvm.org/D14339

llvm-svn: 252628