Remove breaking nsec from android

[bgermann]

On Android targets the current build breaks:

cargo:warning=libgit2/src/iterator.c: In function 'filesystem_iterator_set_current':
cargo:warning=libgit2/src/iterator.c:1464:43: error: 'struct stat' has no member named 'st_ctim'
cargo:warning=  iter->entry.ctime.nanoseconds = entry->st.st_ctime_nsec;
cargo:warning=                                           ^
cargo:warning=libgit2/src/iterator.c:1465:43: error: 'struct stat' has no member named 'st_mtim'
cargo:warning=  iter->entry.mtime.nanoseconds = entry->st.st_mtime_nsec;

[alexcrichton]

Thanks!