net: export genphy_restart_aneg

This patch fixes the following build error caused by
commit ed94493fb38a665cebcf750dfabe8a6dd13e136f
(mv643xx_eth: convert to phylib):

<--  snip  -->

...
  Building modules, stage 2.
  MODPOST 1280 modules
ERROR: "genphy_restart_aneg" [drivers/net/mv643xx_eth.ko] undefined!
...
make[2]: *** [__modpost] Error 1

<--  snip  -->

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>