ofp-actions: Fix build on earlier GCC.

ofp-actions: Fix build on earlier GCC.

GCC4.4 gets confused by anonymous fields + flexible fields, complains:
lib/ofp-actions.h:510: error: flexible array member in otherwise empty
struct
lib/ofp-actions.h:512: error: bit-field ‘build_assert_failed’ width not
an integer constant
lib/ofp-actions.h:514: error: bit-field ‘build_assert_failed’ width not
an integer constant

Fix the problem by specifying the actions length as zero.

Reported-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>