ixp4xx_hss: fix build failure due to missing linux/module.h inclusion

Commit 36a12119 (netprio_cgroup.h:
dont include module.h from other includes) made the following build
error on ixp4xx_hss pop up:

  CC [M]  drivers/net/wan/ixp4xx_hss.o
 drivers/net/wan/ixp4xx_hss.c:1412:20: error: expected ';', ',' or ')'
 before string constant
 drivers/net/wan/ixp4xx_hss.c:1413:25: error: expected ';', ',' or ')'
 before string constant
 drivers/net/wan/ixp4xx_hss.c:1414:21: error: expected ';', ',' or ')'
 before string constant
 drivers/net/wan/ixp4xx_hss.c:1415:19: error: expected ';', ',' or ')'
 before string constant
 make[8]: *** [drivers/net/wan/ixp4xx_hss.o] Error 1

This was previously hidden because ixp4xx_hss includes linux/hdlc.h which
includes linux/netdevice.h which includes linux/netprio_cgroup.h which
used to include linux/module.h. The real issue was actually present since
the initial commit that added this driver since it uses macros from
linux/module.h without including this file.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>