Update erl.mk

Got the following errors:
Cloning gproc / master from git://github.com/esl/gproc.git
git clone -n -- git://github.com/esl/gproc.git /Users/marknijhof/src/smarttv/deps/gproc
fatal: destination path '/Users/marknijhof/src/smarttv/deps/gproc' already exists and is not an empty directory.
make[1]: *** [deps/gproc] Error 128
make: *** [deps/id3as_rtsp] Error 2