message: Add back Manifest::targets_mut
  It is needed by cargo-c, it was removed in
diff: @@ -431,6 +431,10 @@ impl Manifest {
  pub fn targets(&self) -> &[Target] {
  &self.targets
  }
  + // It is used by cargo-c, please do not remove it
  + pub fn targets_mut(&mut self) -> &mut [Target] {
  + &mut self.targets
  + }
  pub fn version(&self) -> &Version {
  self.package_id().version()
  }
  