DLOG_F( ) compilation error

I used Loguru across my project and most of the time it works fine. But in one of the modules, if I use DLOG_F( ) and compiled in Debug mode, a compilation error occurs.

This is the error message given by g++ 5.5.0

```
error: ‘Verbosity_’ is not a member of ‘loguru’
DLOG_F(INFO, "DLOG_F");
```

I checked the value of `LOGURU_DEBUG_LOGGING` and it was set to 1 as expected.

The way I use Loguru is as following:
1. Compile `loguru.hpp` and `loguru.cpp` into a shared lib called `libloguru.so`
2. add `#include ` in my `*.cpp` files where I need to log something.
3. compile my `*.cpp` files and link against `libloguru.so`