opam: add camlp4 to the recommended list

Since OCaml 4.02.0, camlp4 is distributed separately from the main
compiler distribution.  However, it is still required to be installed
alongside the system compiler for the majority of packages.

See Homebrew/homebrew#33596 for more discussion on this topic.

Formula/opam.rb

@@ -4,6 +4,7 @@ class Opam < Formula
   homepage 'https://opam.ocaml.org'
   url 'https://github.com/ocaml/opam/archive/1.2.0.tar.gz'
   sha1 'b7923516a853afe86e8439afd23c0dae5fa8ad57'
+  revision 1
 
   head 'https://github.com/ocaml/opam.git'
 
@@ -16,6 +17,7 @@ class Opam < Formula
 
   depends_on "objective-caml"
   depends_on "aspcud" => :recommended
+  depends_on "camlp4" => :recommended
 
   if build.with? "aspcud"
     needs :cxx11