Fix tools/dsymutil/CMakeLists.txt mis-matching arguments warning (NFC)

CMake Warning (dev) in tools/dsymutil/CMakeLists.txt:
  A logical block opening on the line

    /Users/jiefu/llvm-project/llvm/tools/dsymutil/CMakeLists.txt:42 (if)

  closes on the line

    /Users/jiefu/llvm-project/llvm/tools/dsymutil/CMakeLists.txt:44 (endif)

  with mis-matching arguments.
This warning is for project developers.  Use -Wno-dev to suppress it.