Complete `OCaml.gitignore`

[ilankri]

Reasons for making this change:

• Binary files *.cmt and *.cmti are generated by ocamlc/ocamlopt when invoked using the option -bin-annot.
• These files are used by code inspection tools such as Merlin.

Links to documentation supporting these rule changes:

• https://ocaml.org/manual/comp.html#s:comp-overview