drivers: net: Include new header file in sbni.c

Create a new header file include/net/Space.h which contains
prototype declaration of sbni_probe().

Include the new header file in drivers/net/Space.c and
drivers/net/wan/sbni.c because they use this function.

This eliminates the following warning in wan/sbni.c:
drivers/net/wan/sbni.c:224:12: warning: no previous prototype for ‘sbni_probe’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: David S. Miller <davem@davemloft.net>