Error in META file

Compiling with `Ollvm_llvmgateway` works only while providing `llvm` package to ocamlfind manually, since the META file does not list the dependency:

```
cat $HOME/.opam/4.02.3/lib/ollvm_llvmgateway/META
version = "0.99"
description = "Convert Ollvm AST to Llvm AST."
archive(byte) = "ollvm_llvmgateway.cma"
archive(native) = "ollvm_llvmgateway.cmxa"
```
