sparc: fix warning in userspace header traps.h

Fix following warning:
traps.h:23: extern's make no sense in userspace

Add an ifdef __KERNEL__ block that cover the
extern definition and a few related things that neither
is for userspace.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>