fix IP directed broadcast node declarations

fix IP directed broadcast node declarations

The dev build ('make build') was failing because
ip4_rewrite_bcast() and ip6_rewrite_bcast() are declared static
and were not referenced.

The node functions that were set for directed broadcast rewrite:

ip4_rewrite_bcast_node - ip4_rewrite()
ip6_rewrite_bcast_node - ip6_rewrite()

Changed to:

ip4_rewrite_bcast_node - ip4_rewrite_bcast()
ip6_rewrite_bcast_node - ip6_rewrite_bcast()

The release build ('make build-release') succeeds because there is
a VLIB_NODE_FUNCTION_MULTIARCH() call referencing the function. That
macro is empty for dev builds.

Change-Id: I2a05f00ca43d0eae8ff2e3026991e69917003fe1
Signed-off-by: Matthew Smith <mgsmith@netgate.com>