Upgrade getrandom to 0.2.8

This project was upgraded with external_updater.
Usage: tools/external_updater/updater.sh update rust/crates/getrandom
For more info, check https://cs.android.com/android/platform/superproject/+/master:tools/external_updater/README.md

Test: TreeHugger
Change-Id: Iec2d2bed229e0a061bb7c35f950d84d03424604d
diff --git a/src/bsd_arandom.rs b/src/bsd_arandom.rs
index f26f260..d441212 100644
--- a/src/bsd_arandom.rs
+++ b/src/bsd_arandom.rs
@@ -31,6 +31,7 @@
 }
 
 pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    // getrandom(2) was introduced in FreeBSD 12.0 and NetBSD 10.0
     #[cfg(target_os = "freebsd")]
     {
         use crate::util_libc::Weak;