ofp-actions: Make struct ofpact constant size across implementations.

ofp-actions: Make struct ofpact constant size across implementations.

Before commit c2d936a44fa6 (ofp-actions: Centralize all OpenFlow action
code for maintainability.), struct ofpact was 4 bytes with GCC and Clang,
and 12 bytes with other compilers.  That commit changed struct ofpact so
that it remained 4 bytes with GCC and Clang but shrank to 8 bytes on other
compilers.  An unexpected side effect of that change was that the size
of the pad[] array in struct ofpact_nest shrank to 0 bytes, because that
array padded to a multiple of 8 bytes.  MSVC does not support 0-element
arrays, so this caused a build failure.

This commit fixes the problem by changing struct ofpact so that it is 4
bytes with every compiler.

Reported-by: Alin Serdean <aserdean@cloudbasesolutions.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Alin Serdean <aserdean@cloudbasesolutions.com>