CMake: Fix docs-llvm-man target when clang+llvm is in the same source tree

Summary:
This was broken by r302499.  Configuring with -DLLVM_BUILD_DOCS=ON would
cause the docs-llvm-man target not to be created.

Reviewers: anemet, beanz

Reviewed By: anemet

Subscribers: llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D33146

llvm-svn: 303042