CCN_lite eliminate warning

Compiling ccn_lite yields a warning:
```
RIOT/sys/net/ccn_lite/ccnl-ext-frag.c:711:0: warning: ISO C forbids an empty translation unit [-Wpedantic]
```
when `USE_FRAG` is not set (which is the case by default). This PR introduces a dummy define to convince the compiler that there's something in the unit.

If there's a better way to do so, let me know!

This PR also includes some very small formatting fixes in the same file.