hamachi: Put back RX_CHECKSUM

I deleted it by mistake in the TX_CHECKSUM removal
commit.

Reported-by: Michał Mirosław <mirqus@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/hamachi.c b/drivers/net/hamachi.c
index f5fba73c..a09041a 100644
--- a/drivers/net/hamachi.c
+++ b/drivers/net/hamachi.c
@@ -134,6 +134,7 @@
  *   TCP and UDP packets.  Otherwise the upper layers do the calculation.
  * 3/10/1999 Pete Wyckoff <wyckoff@ca.sandia.gov>
  */
+#define RX_CHECKSUM
 
 /* Operational parameters that usually are not changed. */
 /* Time in jiffies before concluding the transmitter is hung. */