net: skfb: remove obsolete -I cflag

The skfp driver has been moved to drivers/net/fddi/skfp a long time
ago, but we still attempt to include headers from the old location,
which causes a warning when building with W=1:

cc1: error: /git/arm-soc/drivers/net/skfp: No such file or directory [-Werror=missing-include-dirs]
cc1: error: drivers/net/skfp: No such file or directory [-Werror=missing-include-dirs]

Clearly this include directive is not needed any more, so we can
just remove it now.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>