Fix bluetooth_hh_test build

packages/modules/Bluetooth/system/test/common/log_msg.cc:39:3: error: use of undeclared identifier 'va_start'
  va_start(ap, fmt_str);
  ^

packages/modules/Bluetooth/system/test/common/log_msg.cc:41:3: error: use of undeclared identifier 'va_end'
  va_end(ap);
  ^

Fixed by adding "#include <cstdarg>".

Test: presubmit
Change-Id: Id85cc0d069fe7a7b2035923d93adb35f61590fb2