Bug 26434: Fix plugin dirs addition to @INC

Bug 26434: Fix plugin dirs addition to @INC

Plugin dirs defined in koha-conf.xml are added to @INC in order to compile perl code.
Looks like with plack those dirs are added several times.
This may lead to an error "INCLUDE_PATH exceeds 64 directories".

This bug was identified with Carrousel plugin : https://inlibro.com/instructions-carrousel/

Test plan :
1) Enable plack and plugins
2) Look at page about.pl : @INC contains one plugin dir 'var/lib/plugins'
3) Install plugin KitchenSink : https://github.com/bywatersolutions/koha-plugin-kitchen-sink
4) Dont apply patch
5) Use configure on KitchenSink
6) Look at page about.pl : @INC contains server plugin dir 'var/lib/plugins'
7) Apply patch and restart plack
8) Use configure on KitchenSink
9) Look at page about.pl : @INC contains one plugin dir 'var/lib/plugins'

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>