Make thread_priority a shared library

[AndyZe]

This library should be shared so other shared libraries can link it.

I discovered this during this PR: moveit/moveit2#1540

Note that the library I'm linking it to is another shared (aka dynamic) library.

Prior to this PR I get this error:

--- stderr: moveit_servo
/usr/bin/ld: /home/andy/ws_ros2/install/realtime_tools/lib/libthread_priority.a(thread_priority.cpp.o): warning: relocation against `_ZTVSt9basic_iosIcSt11char_traitsIcEE@@GLIBCXX_3.4' in read-only section `.text.unlikely'
/usr/bin/ld: /home/andy/ws_ros2/install/realtime_tools/lib/libthread_priority.a(thread_priority.cpp.o): relocation R_X86_64_PC32 against symbol `_ZTTSt14basic_ifstreamIcSt11char_traitsIcEE@@GLIBCXX_3.4' can not be used when making a shared object; recompile with -fPIC

---

If you don't like the idea of making the thread_priority library SHARED, then another way to fix it is to add this to CMakeLists:

set(CMAKE_CXX_FLAGS "-fPIC ${CMAKE_CXX_FLAGS}")

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[destogl]

Looks good! Thanks!