libc::futimens is not available on Android

[link2xt]

This function is used here:

filetime/src/unix/utimensat.rs

Line 26 in 487c47d

let rc = unsafe { libc::futimens(f.as_raw_fd(), times.as_ptr()) };

But it is not available on Android: https://stackoverflow.com/questions/19374749/how-to-work-around-absence-of-futimes-in-android-ndk

As a result, ndk-make throws an error undefined reference to 'futimens' when trying to build deltachat/deltachat-core-rust#1749 which depends on async-tar which in turn depends on filetime.

[link2xt]

It builds with APP_PLATFORM := android-19, but not with APP_PLATFORM := android-18.