anv: add missing separator to help for intel_clc

intel_clc relies on the special argument '--' for getopt to be given so
it knows when to start expecting purely input files or clang arguments.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19153>

src/intel/compiler/intel_clc.c

@@ -258,7 +258,7 @@ static void
 print_usage(char *exec_name, FILE *f)
 {
    fprintf(f,
-"Usage: %s [options] [clang args | input file]\n"
+"Usage: %s [options] -- [clang args | input file]\n"
 "Options:\n"
 "  -h  --help              Print this help.\n"
 "  -e, --entrypoint <name> Specify the entry-point name.\n"