Add missing overload to ordered_map::find (#5171)

* :bug: add missing overload

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* :art: fix amalgamation

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* :bug: fix typo

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* :rotating_light: fix warning

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

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Signed-off-by: Niels Lohmann <mail@nlohmann.me>