sparc64: fix sparse warnings in kprobes.c

Fix following warnings:
kprobes.c:515:15: warning: symbol 'trampoline_probe_handler' was not declared. Should it be static?
kprobes.c:579:6: warning: symbol 'kretprobe_trampoline_holder' was not declared. Should it be static?

Declare the functions static.
kretprobe_trampoline_holder is magically used without any real
reference so mark is __used, like other arch's do too.

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