Declare dependency on libatomic

[cottsay]

To prefer the system atomics implementation over the fallback implementation in rcutils, declare a dependency on it to ensure it's available on systems that have it.

Current RPM builds for RHEL are using the fallback implementation due to this missing dependency.

Backing rosdep rules: https://github.com/ros/rosdistro/blob/96e2f40ae3400f6d2dc22cce9333f42fa6e4e4be/rosdep/base.yaml#L1856-L1861

[cottsay]

Since this is just a package.xml change, running CI would provide no value.

[clalancette]

Since this is just a package.xml change, running CI would provide no value.

What might provide value, though is to edit the CI scripts to install libatomic1 on Ubuntu, then run CI. That way we would at least know that this change doesn't substantially change anything on Ubuntu (and we should probably merge that change as well, since that is what build.ros2.org is going to be doing when it builds the binary package).

[cottsay]

This is a required dependency for iceoryx_utils, which built successfully in this nightly, which came back green:

So it's already present for CI. It was probably already installed as a recursive dependency prior to the introduction of Iceoryx.