Correct compilation of osnet/dev/systrace/systrace.c under Clang

Mark arguments to dtrace_probe_lookup() with __UNCONST().
The proper fix constifying it causes pollution of const in too many files.

This could be done with -W flags, but they are incompatible between
compilers.

This is a step forward functional MKLLVM=yes HAVE_LLVM=yes build.

Sponsored by <The NetBSD Foundation>

(kamil)