perl: only run finalize hook when necessary

The PERL_FINALIZE_TARGET always runs, even if perl is not installed.
This results in errors from the find command when it fails to find
anything, even when doing "make -s":

>>>   Finalizing target directory
find: `...../output/target/usr/lib/perl5/': No such file or directory
find: `...../output/target/usr/lib/perl5/': No such file or directory
find: `...../output/target/usr/lib/perl5/': No such file or directory

This is not helpful for projects that do not select Perl.  Silence this
by making the hook conditional on package selection.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>