Define mmk_noreturn if __STDC_VERSION__ >= 201112L

[christophebedard]

If CMAKE_C_STANDARD is set to 11 (e.g., globally), then mmk_noreturn is never defined, which results in errors like this:

In file included from /home/user/ros2_ws/build/mimick_vendor/mimick_vendor-prefix/src/mimick_vendor/src/plt.c:25:
/home/user/ros2_ws/build/mimick_vendor/mimick_vendor-prefix/src/mimick_vendor/src/vitals.h:46:13: error: expected ‘;’ before ‘void’
   46 | mmk_noreturn void mmk_panic(const char *, ...);
      |             ^~~~~
      |             ;

_Noreturn is deprecated in C23, but I'm not sure if we should worry about that just yet. See https://en.cppreference.com/w/c/language/_Noreturn.

Relates to #28, but using C99 is not really a great solution.