Compiler: fix compiler_version.ml generation

Remove the \n, it ends up as a literal "\\n" in the generated .ml file
and subsequently breaks the build.

Fixes a regression introduced in commit 45977610 and resolves the
following build error:

    $ make -C compiler
    make: Entering directory `/home/bnoordhuis/src/js_of_ocaml/compiler'
    ocamlfind ocamlopt -package findlib,str -for-pack Compiler -c
    compiler_version.ml
    File "compiler_version.ml", line 1, characters 17-18:
    Error: Illegal character (\\)
    make: *** [compiler_version.cmx] Error 2
    make: Leaving directory `/home/bnoordhuis/src/js_of_ocaml/compiler'