waf: stop clang complaining about variable-length stack arrays

[peterbarker]

Gets rid of all of these:

../../libraries/AP_BattMonitor/AP_BattMonitor_SMBus.cpp:156:18: warning: variable length arrays in C++ are a Clang extension [-Wvla-cxx-extension]
  156 |     uint8_t buff[read_size];    // buffer to hold results
      |                  ^~~~~~~~~

.... which there are a lot of in ArduPilot!