Fix build of llvm-mc-assemble/disassemble-fuzzer

Fix build of llvm-mc-assemble/disassemble-fuzzer

Since these aren't built by default unless building with coverage (and
even then they aren't built for the check target) they've managed to
bit rot a little.

This just fixes the build. See llvm.org/pr34314 for the plan on making
sure these don't bit rot again.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@312011 91177308-0d34-0410-b5e6-96231b3b80d8