compile fails with #define WLED_ENABLE_DMX

when `#define WLED_ENABLE_DMX` is used, the build fails since the cpp-refactoring
```
wled00/wled.cpp: In member function 'void WLED::setup()':
wled00/wled.cpp:216:3: error: 'dmx' was not declared in this scope
   dmx.init(512);        // initialize with bus length
   ^
*** [.pio/build/d1_mini/src/wled.cpp.o] Error 1
```

maybe we should ensure that travis builds all features.