Fix missing version in pin-depends

[Niols]

Without it, all opam commands print:

error  3: File format error in 'pin-depends' at line 44, column 4: while expecting versioned package:
          OpamPackage.of_string

[ygrek]

@jongleb is there a released version of mariadb that we can depend upon?

[Niols]

I checked and sqlgg as of latest master doesn't compile against mariadb.1.3.0, the latest version of the OCaml MariaDB bindings. That being said, that version is from May 2025, so I'm not sure there are releases anymore.

The error:

File "impl/ocaml/mariadb/sqlgg_mariadb.ml", line 169, characters 12-20:
169 |           | `Int64 x -> Int64.to_string x
                  ^^^^^^^^
Error: This pattern matches values of type [? `Int64 of 'a ]
       but a pattern was expected which matches values of type value
       The second variant type does not allow tag(s) `Int64