build: Bump getrandom from 0.2.15 to 0.3.1

Manually bump `getrandom` due to API changes.

Bumps [getrandom](https://github.com/rust-random/getrandom) from 0.2.15 to 0.3.1.
- [Changelog](https://github.com/rust-random/getrandom/blob/master/CHANGELOG.md)
- [Commits](rust-random/getrandom@v0.2.15...v0.3.1)

Signed-off-by: Ruoqing He <heruoqing@iscas.ac.cn>

net_util/src/mac.rs

@@ -80,7 +80,7 @@ impl MacAddr {
     pub fn local_random() -> MacAddr {
         // Generate a fully random MAC
         let mut random_bytes = [0u8; MAC_ADDR_LEN];
-        if let Err(e) = getrandom::getrandom(&mut random_bytes) {
+        if let Err(e) = getrandom::fill(&mut random_bytes) {
             error!(
                 "Error populating MAC address with random data: {}",
                 e.to_string()