powerpc: Fix STACKTRACE=n build

Our skiroot_defconfig doesn't enable FTRACE, and so doesn't get
STACKTRACE enabled either. That leads to a build failure since commit
1614b2b1 ("arch: Make ARCH_STACKWALK independent of STACKTRACE")
made stacktrace.c build even when STACKTRACE=n.

  arch/powerpc/kernel/stacktrace.c: In function ‘handle_backtrace_ipi’:
  arch/powerpc/kernel/stacktrace.c:171:2: error: implicit declaration of function ‘nmi_cpu_backtrace’
    171 |  nmi_cpu_backtrace(regs);
        |  ^~~~~~~~~~~~~~~~~
  arch/powerpc/kernel/stacktrace.c: In function ‘arch_trigger_cpumask_backtrace’:
  arch/powerpc/kernel/stacktrace.c:226:2: error: implicit declaration of function ‘nmi_trigger_cpumask_backtrace’
    226 |  nmi_trigger_cpumask_backtrace(mask, exclude_self, raise_backtrace_ipi);
        |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This happens because our headers haven't defined
arch_trigger_cpumask_backtrace, which causes lib/nmi_backtrace.c not to
build nmi_cpu_backtrace().

The code in question doesn't actually depend on STACKTRACE=y, that was
just added because arch_trigger_cpumask_backtrace() lived in
stacktrace.c for convenience. So drop the dependency on
CONFIG_STACKTRACE, that causes lib/nmi_backtrace.c to build
nmi_cpu_backtrace() etc. and fixes the build.

Fixes: 1614b2b1

 ("arch: Make ARCH_STACKWALK independent of STACKTRACE")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220212111349.2806972-1-mpe@ellerman.id.au