Allow compat_ioctl.c to compile without CONFIG_NET

A small regression appears to have been introduced in the recent patch
"cleanup compat ioctl handling", which was included in Linus' tree after
2.6.20.

siocdevprivate_ioctl() is no longer defined if CONFIG_NET is undefined,
whereas previously it was a dummy function in this case.

This causes compilation with CONFIG_COMPAT but without CONFIG_NET to fail.

fs/compat_ioctl.c: In function `compat_sys_ioctl':
fs/compat_ioctl.c:3571: warning: implicit declaration of function `siocdevprivate_ioctl'

Cc: Christoph Hellwig <hch@lst.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>