chore(rockspec) add missing rockspec entry for

`kong.db.migrations.core.016_280_to_300` which caused the packaging
failure before

Thanks @hutchic @flrgh for pointing out.

kong-2.8.0-0.rockspec

@@ -231,6 +231,7 @@ build = {
     ["kong.db.migrations.core.013_220_to_230"] = "kong/db/migrations/core/013_220_to_230.lua",
     ["kong.db.migrations.core.014_230_to_270"] = "kong/db/migrations/core/014_230_to_270.lua",
     ["kong.db.migrations.core.015_270_to_280"] = "kong/db/migrations/core/015_270_to_280.lua",
+    ["kong.db.migrations.core.016_280_to_300"] = "kong/db/migrations/core/016_280_to_300.lua",
     ["kong.db.migrations.operations.200_to_210"] = "kong/db/migrations/operations/200_to_210.lua",
     ["kong.db.migrations.operations.210_to_211"] = "kong/db/migrations/operations/210_to_211.lua",
     ["kong.db.migrations.operations.212_to_213"] = "kong/db/migrations/operations/212_to_213.lua",