fix: ensure /usr/lib/multiflexi-database/seeds/ dir is created in package

dh only installs files listed in .install; if the seeds/ directory is
absent on the installed system phinx silently omits it from seed paths
and 'phinx seed:run -s EncryptionKeysSeeder' fails with "class does
not exist". Adding the dir to multiflexi-migrations.dirs guarantees it
exists even before any seed files are copied.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Vitexus

debian/multiflexi-migrations.dirs

@@ -1 +1,2 @@
 /etc/multiflexi
+/usr/lib/multiflexi-database/seeds