Add a missing using to get the floating abs on OSX

[sherm1]

A missing using std::abs caused OSX to choose the integer absolute value rather than floating.

Fixes #4955.

---

This change is 

[sherm1]

+@liangfok for review please. Can you verify that this works on the platform that failed in #4955?

---

Review status: 0 of 1 files reviewed at latest revision, all discussions resolved.

---

Comments from Reviewable

[liangfok]

Sure. Checking now.

---

Review status: 0 of 1 files reviewed at latest revision, all discussions resolved.

---

Comments from Reviewable

[liangfok]

@drake-jenkins-bot mac-clang-ninja-experimental please.

[liangfok]

The warning did not appear in the console log of https://drake-jenkins.csail.mit.edu/job/mac-clang-ninja-experimental/232/. I'm still waiting for my local machine to finish building.

[liangfok]

cpplint warning:

Warning	
drake/multibody/benchmarks/cylinder_torque_free_analytical_solution/test/cylinder_torque_free_analytical_solution.cc:116: warning: Line ends in whitespace.  Consider deleting these extra spaces.  [whitespace/end_of_line] [4]

[liangfok]

I confirm that this fixes the warning on both CI and my local machine. Thanks!

---

Reviewed 1 of 1 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

---

Comments from Reviewable

[sherm1]

Thanks, Liang!

---

Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

---

Comments from Reviewable