Fix build in oss

Summary:
Another very helpful clippy suggestion.

```
error: incorrect implementation of `partial_cmp` on an `Ord` type
  --> gazebo/src/phantom.rs:75:1
   |
75 | /  impl<T: ?Sized> PartialOrd<PhantomDataInvariant<T>> for PhantomDataInvariant<T> {
76 | |      fn partial_cmp(&self, other: &PhantomDataInvariant<T>) -> Option<std::cmp::Ordering> {
   | | __________________________________________________________________________________________-
77 | ||         self.0.partial_cmp(&other.0)
78 | ||     }
   | ||_____- help: change this to: `{ Some(self.cmp(other)) }`
79 | |  }
   | |__^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#incorrect_partial_ord_impl_on_ord_type
   = note: `#[deny(clippy::incorrect_partial_ord_impl_on_ord_type)]` on by default```

Reviewed By: krallin

Differential Revision: D48941842

fbshipit-source-id: 914da37629fc16b3324d9ae31321e4377c7f0fc1

Author: stepancheg

gazebo/gazebo/src/phantom.rs

@@ -74,6 +74,6 @@ impl<T: ?Sized> PartialEq<PhantomDataInvariant<T>> for PhantomDataInvariant<T> {
 
 impl<T: ?Sized> PartialOrd<PhantomDataInvariant<T>> for PhantomDataInvariant<T> {
     fn partial_cmp(&self, other: &PhantomDataInvariant<T>) -> Option<std::cmp::Ordering> {
-        self.0.partial_cmp(&other.0)
+        Some(self.cmp(other))
     }
 }