fix libcoap checkout target

Fixes the following error:
```
make[1]: Entering directory '/foo/RIOT/pkg/libcoap'
git clone http://git.code.sf.net/p/libcoap/code /foo/RIOT/pkg/libcoap/libcoap && \
	cd /foo/RIOT/pkg/libcoap/libcoap && git checkout 89acaa6775ca3a4aedea510557e6b9c0c01fa5db
fatal: destination path '/foo/RIOT/pkg/libcoap/libcoap' already exists and is not an empty directory.
Makefile:25: recipe for target '/foo/RIOT/pkg/libcoap/libcoap' failed
```

Test case:
run `make -B clean all` twice with https://github.com/authmillenon/projects/tree/libcoap