kbuild: ignore cache modifiers for generating the tags files

With this patch I'm able to find the definition of _xmit_lock defined in
include/linux/netdevice.h as follows:

	struct net_device {
		...
		spinlock_t _xmit_lock ____cacheline_aligned_in_smp;
	}

Otherwise this counts as definition of ____cacheline_aligned_in_smp.

Signed-off-by: Uwe Kleine-König <Uwe.Kleine-Koenig@digi.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>