devel/p5-AnyEvent-Subprocess: add small fix

There is a bug in AnyEvent::Subprocess since 2015,
which requires a one line patch and unfortunately
the authors of AnyEvent::Subprocess still did not fix it.
This bug produces a warning on resent versions of Moose during startup:

Passing a list of values to enum is deprecated. Enum values should be wrapped
in an arrayref.
at /usr/local/lib/perl5/site_perl/AnyEvent/Subprocess/Types.pm line 42.

See also: https://github.com/zhmylove/pwsvpn

Reported by: Sergei Zhmylove