meson: standardize .so version to major.minor.patch

meson: standardize .so version to major.minor.patch

This `version` field defines the filename for the .so.
The plan .so as well as .so.$major are always symlinks to this.

Unless I'm mistaken, only the major is ever used, so this shouldn't
matter, but for consistency with autotools (and in case it does matter),
let's always have all 3 major.minor.patch components.

(The soname isn't affected, and is always .so.$major)

Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>