Fix compilation for newer clang

[Tobias-Fischer]

No description provided.

[mintar]

Thanks for your PR!

The pre-commit check in the CI is currently failing (see above). You can reproduce this locally by following the pre-commit instructions in the README. I've run catkin_lint with the additional --explain flag locally:

$ catkin_lint --explain rviz_imu_plugin/
rviz_imu_plugin: CMakeLists.txt(16): warning: variable CMAKE_CXX_FLAGS is modified
     * You have appended extra data to a critical CMake variable. This
     * might break the build on different systems or affect the global
     * catkin workspace in unintended ways.
     * You can ignore this problem with --ignore critical_var_append
rviz_imu_plugin: CMakeLists.txt(17): warning: variable CMAKE_C_FLAGS is modified
catkin_lint: checked 1 packages and found 2 problems
catkin_lint: option -W2 will show 5 additional notices

I believe the way to fix this is to use add_compile_options or even better target_compile_options instead of SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} [...] and SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} [...]. Could you try this?