powerpc/powermac: Add missing include of header pmac.h

powerpc/powermac: Add missing include of header pmac.h

The header `pmac.h` was not included, leading to the following warnings,
treated as error with W=1:

  arch/powerpc/platforms/powermac/time.c:69:13: error: no previous prototype for ‘pmac_time_init’ [-Werror=missing-prototypes]
  arch/powerpc/platforms/powermac/time.c:207:15: error: no previous prototype for ‘pmac_get_boot_time’ [-Werror=missing-prototypes]
  arch/powerpc/platforms/powermac/time.c:222:6: error: no previous prototype for ‘pmac_get_rtc_time’ [-Werror=missing-prototypes]
  arch/powerpc/platforms/powermac/time.c:240:5: error: no previous prototype for ‘pmac_set_rtc_time’ [-Werror=missing-prototypes]
  arch/powerpc/platforms/powermac/time.c:259:12: error: no previous prototype for ‘via_calibrate_decr’ [-Werror=missing-prototypes]
  arch/powerpc/platforms/powermac/time.c:311:13: error: no previous prototype for ‘pmac_calibrate_decr’ [-Werror=missing-prototypes]

The function `via_calibrate_decr` was made static to silence a warning.

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>