Always link rmf_traffic against the Threads target

[mxgrey]

open-rmf/rmf_demos#132 reported that their machine failed to link rmf_traffic against pthread. In theory this PR should fix that problem.

[codecov]

Codecov Report

Merging #74 (517e27b) into main (50f3211) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #74   +/-   ##
=======================================
  Coverage   23.14%   23.14%           
=======================================
  Files         370      370           
  Lines       30583    30583           
  Branches    14262    14262           
=======================================
  Hits         7079     7079           
  Misses      14103    14103           
  Partials     9401     9401           

Flag	Coverage Δ
tests	23.01% <ø> (ø)

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[cottsay]

I believe that this PR will solve one of the issues preventing rmf_traffic_examples from successfully building RPM packages for RHEL: https://build.ros2.org/view/Rbin_rhel_el864/job/Rbin_rhel_el864__rmf_traffic_examples__rhel_8_x86_64__binary/

EDIT: And also rmf_battery builds: https://build.ros2.org/view/Rbin_rhel_el864/job/Rbin_rhel_el864__rmf_battery__rhel_8_x86_64__binary/