ofp-actions: Relax build assertion condition for ofpact_nest struct.

ofp-actions: Relax build assertion condition for ofpact_nest struct.

struct ofpact has enums that are packed in case of __GNUC__.
This packing does not occur for visual studio. For 'struct ofpact_nest',
we are currently expecting that "struct ofpact actions[]" has an offset of
8 bytes.  This condition won't be true in compilers where enums are
not packed.

It is good enough if struct ofpact actions[] starts at an offset which is
a multiple of OFPACT_ALIGNTO.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>