chore: Fix an unused import warning

[maspe36]

Put use rustflags; next to its usage to prevent a build warning.

   warning: the item `rustflags` is imported redundantly
    --> rclrs/build.rs:24:13
     |
  24 |         use rustflags;
     |             ^^^^^^^^^ the item `rustflags` is already defined here
     |
     = note: `#[warn(unused_imports)]` on by default
  
  warning: unused import: `rustflags`
    --> rclrs/build.rs:24:13
     |
  24 |         use rustflags;
     |             ^^^^^^^^^
  
  ---
  Finished <<< rclrs [9.19s]
  --- stderr: rclrs
  warning: the item `rustflags` is imported redundantly
    --> rclrs/build.rs:24:13
     |
  24 |         use rustflags;
     |             ^^^^^^^^^ the item `rustflags` is already defined here
     |
     = note: `#[warn(unused_imports)]` on by default
  
  warning: unused import: `rustflags`
    --> rclrs/build.rs:24:13
     |
  24 |         use rustflags;
     |             ^^^^^^^^^
  
  ---