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ARROW-14598: [C++][Flight] Fix protoc generation dependency for example

* DEPENDS variable name has a typo.
* ARROW_PROTOBUF_PROTOC doesn't depend on protobuf_ep with bundled Protobuf

Closes #11615 from kou/cpp-flight-example-fix-dependency

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>