trace: fix build with gcc9

trace: fix build with gcc9

While I've not observed this myself, gcc 9 (imo validly) reportedly may
complain

trace.c: In function '__trace_hypercall':
trace.c:826:19: error: taking address of packed member of 'struct <anonymous>' may result in an unaligned pointer value [-Werror=address-of-packed-member]
  826 |     uint32_t *a = d.args;

and the fix is rather simple - remove the __packed attribute. Introduce
a BUILD_BUG_ON() as replacement, for the unlikely case that Xen might
get ported to an architecture where array alignment higher that that of
its elements.

Reported-by: Martin Liška <martin.liska@suse.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>