[S390] Simplify stack trace.

sparse gives us a few of these:
stacktrace.c:69:38: warning: incorrect type in argument 2
                    (different signedness)
stacktrace.c:69:38:    expected unsigned int *skip

Just get rid of the 'skip' argument since it is contained in the
struct stack_trace that gets passed anyway.

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