[S390] kretprobe_trampoline_holder() in wrong section.

kretprobe_trampoline_holder() is in kprobes section but used to
register a kprobe in arch_init_kprobes(). Hence register_kprobe()
and therefore arch_init_kprobes() will fail.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>