journal: disable -Waddress-of-packed-member under clang

journal: disable -Waddress-of-packed-member under clang

clang warns about a few sites like this:
../src/journal/journal-file.c:1780:48: warning: taking address of packed member 'entry_offset' of class or structure 'DataObject' may result in an unaligned pointer value [-Waddress-of-packed-member]
                                              &o->data.entry_offset,
                                               ^~~~~~~~~~~~~~~~~~~~
but DataObject.entry_offset will always be 8-byte aligned as long as
the DataObject structure is aligned. Similarly in other cases, the
field is always aligned. Let's just silence the warning to avoid noise.

gcc does not know -Waddress-of-packed-member, and would warn about an unknown
warning, so we need to conditionalize on __clang__.