lib/perl5db.pl: Restore noop lock prototype

lib/perl5db.pl: Restore noop lock prototype

cde405a6b9b86bd8110f63531b42d89590a4c56e removed the lock prototype
"because it's already a do-nothing weak keyword without threads".
However, that causes "perl -d threaded-script.pl" to complain

    lock can only be used on shared values at /usr/share/perl/5.20/perl5db.pl line 4101.
    BEGIN failed--compilation aborted at threaded-script.pl line 2.
    lock can only be used on shared values at /usr/share/perl/5.20/perl5db.pl line 2514.
    END failed--call queue aborted at threaded-script.pl line 2.
    Unbalanced scopes: 3 more ENTERs than LEAVEs

because threaded-script.pl's importing of threads::shared enable's
lock()'s non-noop behavior.  Restoring the lock() prototype fixes the
inconsistency between lock() and share() usage.

Signed-off-by: James McCoy <vega.james@gmail.com>