Libnet:  remove unused header

Do not include <rpc/rpc.h> as it is unused.  While this is normally
harmless, glibc is dropping support for rpc as of 2.14 [1] and ipv6 is
only supported in ti-rpc.

This fixes building on distros that have not manually patched rpc
support back into glibc.

1. http://sourceware.org/bugzilla/show_bug.cgi?id=12949