powerpc/powermac: mark smp_psurge_{give,take}_timebase static

These functions are only called locally and should be static like the
other corresponding functions are:

arch/powerpc/platforms/powermac/smp.c:416:13: error: no previous prototype for 'smp_psurge_take_timebase' [-Werror=missing-prototypes]
  416 | void __init smp_psurge_take_timebase(void)
      |             ^~~~~~~~~~~~~~~~~~~~~~~~
arch/powerpc/platforms/powermac/smp.c:432:13: error: no previous prototype for 'smp_psurge_give_timebase' [-Werror=missing-prototypes]
  432 | void __init smp_psurge_give_timebase(void)
      |             ^~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231108125843.3806765-20-arnd@kernel.org