journal: move _packed_ attribute to the end

journal: move _packed_ attribute to the end

GCC manual states that "For an enum, struct or union type, you may
specify attributes either between the enum, struct or union tag and
the name of the type, or just past the closing curly brace of the
definition. The former syntax is preferred." This means that the
attribute should not be located before 'struct'. Putting it between
'struct' and the name seems cluttered. Putting it at the end seems
most readable.

This avoids clang warnings.