libfdisk: fix printf format modifier

libfdisk: fix printf format modifier

libfdisk/src/script.c: In function ‘fdisk_script_read_context’:
libfdisk/src/script.c:452:33: error: format ‘%zu’ expects argument of type ‘size_t’, but argument 4 has type ‘long unsigned int’ [-Werror=format=]
   snprintf(buf, sizeof(buf), "%zu", fdisk_get_grain_size(cxt));
                                 ^

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>