powerpc, ftrace: fix compile error when modules not configured

Michael Neuling reported a compile bug when dynamic ftrace was
configured in and modules were not. This was due to the ftrace
code referencing module specific structures.

Reported-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>