Register the ORFEntry IP address as an IPv4 address.

We're adding it with proto_tree_add_ipv4(), so that it's displayed as,
and can be tested as, an IPv4 address.  That means that the field for it
must be of type FT_IPv4, *even though it's calculated from an 8-bit
prefix value* - you can't make it an FT_UINT8.

Change-Id: I7d668fb2a707cdda4ad228afcbdcaf47ee6cba1b
Reviewed-on: https://code.wireshark.org/review/5960
Reviewed-by: Guy Harris <guy@alum.mit.edu>