tests/mkcert.gmk: Fix makefile bug in snakeoil cert generator
tests/mkcert.gmk: Fix makefile bug in snakeoil cert generator

Just in case someone needs it.

Message-Id: <24b7af9c271b158ca752709419ab20958e1dfeb4@scylladb.com>

dreamsxin/seastar,syuu1228/seastar,scylladb/seastar,dreamsxin/seastar,cloudius-systems/seastar,syuu1228/seastar,scylladb/seastar,cloudius-systems/seastar,avikivity/seastar,kjniemi/seastar,kjniemi/seastar,cloudius-systems/seastar,avikivity/seastar,kjniemi/seastar,syuu1228/seastar,dreamsxin/seastar,scylladb/seastar,avikivity/seastar