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
Merge pull request #578 from LudwigOrtmann/libcoap_makefile

fix libcoap checkout target