src/oping.c: Fix a compiler warning about uninitialized variables.

src/oping.c: Fix a compiler warning about uninitialized variables.

Some verions of GGC don't notice that usage_qos_exit() won't return and
complain about the variables "prec" and "class" being used uninitalized.
This commit adds the "noreturn" attribute to that function and
initializes the variables. Thanks to James Bromberger for reporting this
problem.