get_reviewer.pl: improve portability

get_reviewer.pl: improve portability

Not all package managers / users will install perl into /usr/bin,
but /usr/bin/env /should/ always be present.

Using /usr/bin/env means that we can't give the -w argument to Perl,
so I added `use warnings' in the script.

Reviewed-by: Rob Clark <robclark@freedesktop.org>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>