conf-perl: add MSYS2 depext declaration

[smorimoto]

The conf-perl package currently has depext declarations for various platforms including Cygwin, but is missing one for MSYS2:

depexts: [
  ["perl"] {os-family = "debian"}
  ...
  ["perl"] {os-distribution = "cygwin"}
  # Missing: ["perl"] {os-distribution = "msys2"}
]

On MSYS2, perl is available as the perl package via pacman, so the fix should be straightforward:

["perl"] {os-distribution = "msys2"}

This was discovered whilst adding MSYS2 support to ocaml/setup-ocaml. Without this declaration, packages that depend on conf-perl cannot have perl automatically installed via depext when using opam with an MSYS2 environment.

Related: #29575 (conf-m4)