Remove config.h with `make clean`

I got a compilation error after pulling the changes merged in #117:
```
settings.c: In function `load_settings`:
settings.c:143:8: error: `show_indicators` undeclared (first use in this function)
        show_indicators,
        ^
settings.c:143:8: note: each undeclared identifier is reported only once for each function it appears in
```

Turns out that a new variable has been declared in `config.def.h` but I already had a `config.h` generated from it before and the Makefile doesn't regenerates it when it already exists.

I think it is appropriate to remove `config.h` with `make clean` since it's a byproduct of the make process and depends on `config.def.h`.