Add GNU Multiple Precision library as bazel dep

This currently requires gmp to be installed.

If you see some error like 
```

external/pbc/arith/fieldquadratic.c:14:10: fatal error: gmp.h: No such file or directory
--
  | #include <gmp.h>
  | ^~~~~~~

```

Then install gmp with `apt-get install libgmp3-dev` for now.

Ideally, we should not depend on this to be installed and provide this via cc_library like we have done with pbc. 