powerpc/configs/powernv: Add IGB=y

Some powernv machines use IGB for networking, so build the driver in to
enable net booting such machines.

Suggested-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230420052149.1328094-1-mpe@ellerman.id.au