soc: posix: inf_clock: add C++ support to NATIVE_TASK

Explicitly define function pointer as (void*)() and not (void*).
In C this cast is done implicitly, but C++ does not allow it.

Also, the const is moved to the correct location. Now it's a constant
pointer to a function(void) that returns void, instead of a pointer
to a function(void) returning (const void).

Signed-off-by: Hein Wessels <heinwessels93@gmail.com>