Fix PERL5LIB setting bug.

Whoops! The error became apparent to me when I installed 5.17.10 to see
if I could reproduce the issue reported by myra
(https://github.com/kablamo/VimDebug/issues/32). My previous
environment, with 5.16.1, already had the v0.903 version of Vim::Debug
installed, so the erroneous PERL5LIB setting had no apparent ill effect
(none of the recent modifications I made involved the .pm files).