net: dsa: sja1105: suppress -Wmissing-prototypes in sja1105_vl.c

Newer C compilers are complaining about the fact that there are no
function prototypes in sja1105_vl.c for the non-static functions.
Give them what they want.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>