.gitignore: ignore 'benchmarks' folder

.gitignore: ignore 'benchmarks' folder

Now, you can clone the benchmarks repository under the name 'benchmarks'
and have it compile-ready (but ignored by this repository) with:

  git clone <URI/model-checker-benchmarks.git> benchmarks
  cd benchmarks
  make

Future updates can be pulled in via a 'git pull' from that directory:

  cd benchmarks
  git pull