generator.ml: avoid warnings in generated ocaml/guestfs_c_actions.c

* src/generator.ml: Emit prototypes for ocaml_guestfs_* functions,
to avoid warnings from gcc -Wmissing-prototypes.  Normally we'd put
these somewhere else, but in this unusual case, they're not needed
anywhere else.  Handle the >5-argument case, too, for these:
ocaml_guestfs_test0_byte, ocaml_guestfs_sfdisk_byte,
ocaml_guestfs_sfdisk_N_byte.