[ARM] fix pnx4008 build errors

include/linux/i2c-pnx.h was missed when moving the include files.
Fix it now; it doesn't really need to include mach/i2c.h at all.
Successfully build tested with pnx4008_defconfig, which had
failed in linux-next.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>