lavf/isom: Make auxiliary_offsets consistently uint64_t.

Fixes a compilation warning if size_t != uint64_t:
libavformat/mov.c: In function ‘mov_read_saio’:
libavformat/mov.c:6207:45: warning: assignment from incompatible pointer type [-Wincompatible-pointer-types]
         encryption_index->auxiliary_offsets = auxiliary_offsets;
                                             ^