glsl: use util_snprintf()

Instead of plain snprintf(). To fix the MSVC 2013 build.

Fixes: 6ff0c6f4 ("gallium: move ddebug, noop, rbug, trace to auxiliary to improve build times")
Cc: Marek Olšák <marek.olsak@amd.com>
Cc: Brian Paul <brianp@vmware.com>
Cc: Roland Scheidegger <sroland@vmware.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Brian Paul <brianp@vmware.com>