ARM: sa1100: don't warn about mach/ide.h

This warning has existed since before the start of (git) history.
Apparently nobody has bothered to fix it in a long time, and
this is unlikely to change. Note that the file that the warning
refers to has moved to a different location and was subsequently
deleted in 2008.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Russell King <linux@arm.linux.org.uk>