Add missing Makefile rule for r8152 driver

In the current kernel the r8152 driver is *never* built because of a missing rule in drivers/net/Makefile, despite being selected as built-in or module in the .config file. There is no error message or warning to indicate that the driver isn't built. This change adds the rule and lets the driver build.

Tested as built-in and module for 3.15.8.

Signed-off by: JF Le Fillatre <jflf-kernel@gmx.com>

Signed-off-by: David S. Miller <davem@davemloft.net>