[lldb][test] Fix -Wctad-maybe-unsupported in AlarmTest.cpp (NFC)

llvm-project/lldb/unittests/Host/AlarmTest.cpp:49:7:
error: 'lock_guard' may not intend to support class template argument deduction [-Werror,-Wctad-maybe-unsupported]
      std::lock_guard guard(m);
      ^