Add JTAGSignature to backward compatibility exports

JTAGSignature is used by chipflow-examples mcu_soc project but was
missing from the backward compatibility stub exports.

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: robtaylor

chipflow_lib/platforms/__init__.py

@@ -24,6 +24,7 @@
     GPIOSignature,
     I2CSignature,
     InputIOSignature,
+    JTAGSignature,
     OutputIOSignature,
     QSPIFlashSignature,
     SPISignature,