x86: avoid multiple declaration of kstack_depth_to_print

Impact: cleanup

asm/stacktrace.h is more appropriate so removing other 2 declarations.

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Cc: Neil Horman <nhorman@tuxdriver.com>
LKML-Reference: <1239695308.3033.34.camel@ht.satnam>
Signed-off-by: Ingo Molnar <mingo@elte.hu>