`int_range::make_singular()` accepts a single `int` as its parameter,
so there is no need to brace the paramter with `{}`. this helps to silence
the warning from Clang, like:
```
/home/kefu/dev/scylladb/test/perf/perf_fast_forward.cc:1396:63: error: braces around scalar initializer [-Werror,-Wbraced-scalar-init]
check_no_disk_reads(test(int_range::make_singular({100}))),
^~~~~
```
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closes#12903
50f68fe475