block: partitions: only define function mac_fix_string for CONFIG_PPC_PMAC

The helper function mac_fix_string is only required with CONFIG_PPC_PMAC,
add #if CONFIG_PPC_PMAC and #endif around the function.

Cleans up clang scan build warning:
block/partitions/mac.c:23:20: warning: unused function 'mac_fix_string' [-Wunused-function]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Link: https://lore.kernel.org/r/20240308133921.2058227-1-colin.i.king@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>