Missing XFS_SUPER_MAGIC for android target

[MrCroxx]

Target Triple: *-linux-android
API: https://android.googlesource.com/platform/bionic/+/master/libc/include/sys/vfs.h#105

Bionic libc for Android has exported XFS_SUPER_MAGIC, but rust libc hasn't.

I'm not that familiar with Android, but when I was fixing the same problem for rust lib nix, I found that rust libc missed the constant for android target and I checked the bionic libc source code. I will send a PR if there is no problem.