schema: fix build issues with libstdc++ 12
schema: fix build issues with libstdc++ 12

Switch from using `std::map::insert` to `std::map::emplace`
in the `get_sharder()` function, since we are constructing
a temporary value anyway.

Also, use `std::make_pair` instead of initializer list because
for some reason Clang 13 w/ libstdc++ 12 argues about not
being able to find a suitable overload.

Signed-off-by: Pavel Solodovnikov <2cf5887fafb117b7ff3e29b9720a7305c3313494@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla