fix the multiple definitions of lexical casts methods

[saikishor]

Fixes the issue caused in the PR : ros-controls/ros2_control_demos#428

When you have multiple files using the lexical casts header and they create the same library/executable, they show this multiple definitions error

--- stderr: ros2_control_demo_example_14
/usr/bin/ld: CMakeFiles/ros2_control_demo_example_14.dir/hardware/rrbot_sensor_for_position_feedback.cpp.o: in function `hardware_interface::stod(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)':
rrbot_sensor_for_position_feedback.cpp:(.text+0x3a): multiple definition of `hardware_interface::stod(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'; CMakeFiles/ros2_control_demo_example_14.dir/hardware/rrbot_actuator_without_feedback.cpp.o:rrbot_actuator_without_feedback.cpp:(.text+0xb): first defined here
/usr/bin/ld: CMakeFiles/ros2_control_demo_example_14.dir/hardware/rrbot_sensor_for_position_feedback.cpp.o: in function `hardware_interface::parse_bool(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)':
rrbot_sensor_for_position_feedback.cpp:(.text+0x1cb): multiple definition of `hardware_interface::parse_bool(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'; CMakeFiles/ros2_control_demo_example_14.dir/hardware/rrbot_actuator_without_feedback.cpp.o:rrbot_actuator_without_feedback.cpp:(.text+0x19c): first defined here
collect2: error: ld returned 1 exit status
gmake[2]: *** [CMakeFiles/ros2_control_demo_example_14.dir/build.make:257: libros2_control_demo_example_14.so] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:137: CMakeFiles/ros2_control_demo_example_14.dir/all] Error 2
gmake: *** [Makefile:146: all] Error 2
---
```

[codecov]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (2d82de8) 47.89% compared to head (474d424) 47.87%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1281      +/-   ##
==========================================
- Coverage   47.89%   47.87%   -0.03%     
==========================================
  Files          41       41              
  Lines        3451     3453       +2     
  Branches     1876     1878       +2     
==========================================
  Hits         1653     1653              
  Misses        444      444              
- Partials     1354     1356       +2     

Flag	Coverage Δ
unittests	47.87% <29.41%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...controller_interface/controller_interface_base.hpp	33.33% <ø> (ø)
.../include/controller_manager/controller_manager.hpp	18.18% <ø> (ø)
...roller_interface/src/controller_interface_base.cpp	48.14% <0.00%> (ø)
controller_manager/src/controller_manager.cpp	39.15% <16.66%> (-0.07%)	⬇️
hardware_interface/src/lexical_casts.cpp	40.00% <40.00%> (ø)