util: Fix include for htonl().

Commit 526a7c85d11dc "util: Add be32_prefix_mask()." added an include for
byte-order.h into util.h, which could cause link failures if users of
libopenvswitch defined their own version of htonll(). Change the
include, as only htonl() is needed and arpa/inet.h provides this.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>