dtrace: Add missing const attributes to types in glib_probes.d
This eliminates some ‘discards const qualifier’ warnings when compiling on Solaris with --enable-dtrace.
See the log files in #1493 (comment 299037).
Signed-off-by: Philip Withnall withnall@endlessm.com
Not even compile tested. If you merge this, you get to keep both pieces.