powerpc/ftrace: Fix indentation in ftrace.h

Replace seven spaces with a tab character to fix an indentation issue
reported by the kernel test robot.
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202311221731.alUwTDIm-lkp@intel.com/Signed-off-by: Naveen N Rao <naveen@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/9f058227bd9243f0842786ef7228d87ab10d29f6.1702045299.git.naveen@kernel.org