net/smc: remove unreferenced header in smc_loopback.h file

Because linux/err.h is unreferenced in smc_loopback.h file, so
remove it.
Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: D. Wythe <alibuda@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/smc/smc_loopback.h

@@ -15,7 +15,6 @@
 #define _SMC_LOOPBACK_H
 
 #include <linux/device.h>
-#include <linux/err.h>
 #include <net/smc.h>
 
 #if IS_ENABLED(CONFIG_SMC_LO)