Fix rand_jitter on older versions of rust

In at least rust 1.26 on osx, rand_jitter has a compile error due to libc not being imported. I'm guessing this was fixed as part of the 2018 edition, which is why this isn't showing up on the stable build.

Author: erickt

rand_jitter/src/platform.rs

@@ -21,6 +21,8 @@ pub fn get_nstime() -> u64 {
 
 #[cfg(any(target_os = "macos", target_os = "ios"))]
 pub fn get_nstime() -> u64 {
+    use libc;
+    
     // On Mac OS and iOS std::time::SystemTime only has 1000ns resolution.
     // We use `mach_absolute_time` instead. This provides a CPU dependent
     // unit, to get real nanoseconds the result should by multiplied by