jextract: Fix duplicate rawValue generation for enums

[sidepelican]

Currently, providing an enum that explicitly defines rawValue causes the generator to produce duplicate rawValue implementations.

public enum DeliveryStage : Swift.String, Swift.Sendable & Swift.Codable {
  case schedule
  case delivered
  public init?(rawValue: Swift.String)
  public typealias RawValue = Swift.String
  public var rawValue: Swift.String {
    get
  }
}

error: invalid redeclaration of 'Java_com_example_swift_DeliveryStage__00024getRawValue__J(environment:thisClass:selfPointer:)'

There was a simple logic error in the generator.

[ktoso]

Thank you!