net: Drop duplicate include of dm.h in pcnet.c

This file includes the header twice. Drop the second one.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/net/pcnet.c b/drivers/net/pcnet.c
index 5595608..ad5ac66 100644
--- a/drivers/net/pcnet.c
+++ b/drivers/net/pcnet.c
@@ -10,7 +10,6 @@
 #include <cpu_func.h>
 #include <dm.h>
 #include <log.h>
-#include <dm.h>
 #include <malloc.h>
 #include <memalign.h>
 #include <net.h>