opam.export: bump ocaml-compiler-libs v0.12.{3->4}

Author: glyh

opam.export

@@ -180,7 +180,7 @@ installed: [
   "num.1.1"
   "ocaml.4.14.2"
   "ocaml-base-compiler.4.14.2"
-  "ocaml-compiler-libs.v0.12.3"
+  "ocaml-compiler-libs.v0.12.4"
   "ocaml-config.2"
   "ocaml-migrate-parsetree.2.3.0"
   "ocaml-options-vanilla.1"