Declare dependency on libatomic (#338)

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.

Signed-off-by: Scott K Logan <logans@cottsay.net>

package.xml

@@ -12,6 +12,8 @@
   <buildtool_depend>ament_cmake_ros</buildtool_depend>
   <buildtool_depend>python3-empy</buildtool_depend>
 
+  <depend>libatomic</depend>
+
   <test_depend>ament_cmake_gmock</test_depend>
   <test_depend>ament_cmake_gtest</test_depend>
   <test_depend>ament_cmake_pytest</test_depend>