scripts: get_feat.pl: use /usr/bin/env to find perl

I tried running `make pdfdocs` on NixOS, but it failed because
get_feat.pl uses a shebang line with /usr/bin/perl, and that file path
doesn't exist on NixOS. Using the more portable /usr/bin/env perl fixes
the problem.

Signed-off-by: David Reaver <me@davidreaver.com>
Link: https://lore.kernel.org/r/20220625211548.1200198-1-me@davidreaver.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>