openrisc/traps: Declare file scope symbols as static

Sparse was reporting the following warnings:

    arch/openrisc/kernel/traps.c:37:5: warning: symbol 'kstack_depth_to_print' was not declared. Should it be static?
    arch/openrisc/kernel/traps.c:39:22: warning: symbol 'lwa_addr' was not declared. Should it be static?
    arch/openrisc/kernel/traps.c:41:6: warning: symbol 'print_trace' was not declared. Should it be static?

The function print_trace and local variables kstack_depth_to_print and
lwa_addr are not used outside of this file.  This patch marks them as
static.

Signed-off-by: Stafford Horne <shorne@gmail.com>