cc110x_ng: build cc110x_spi for avsextrem (bugfix)

Building the cc110x_ng driver fails when setting BOARD=avsextrem. 
You can easily check this by adding the following line to examples/hello-world/Makefile:
```
USEMODULE += cc110x_ng
```
and then building hello-world for avsextrem:
```
BOARD=avsextrem make
```
This should fail with a message similar to this:
```
arm-none-eabi-gcc: error: /home/philipp/RIOT/examples/hello-world/bin/avsextrem/cc110x_spi.a: No such file or directory
```

This PR would fix this problem.

BTW: Why is cc110x and not cc110x_ng the default transceiver driver for avsextrem? I find this a little bit strange since the AVS-Extrem boards are nearly identical to the msba2 boards and yet they use different default transceiver drivers...