der_derive: add workaround for `syn` v2.0.73 #1517

Job	Run time
build (1.70.0, thumbv7em-none-eabi)	29s
build (1.70.0, wasm32-unknown-unknown)	31s
test (1.70.0)	1m 32s
build (stable, thumbv7em-none-eabi)	30s
test (stable)	1m 16s
build (stable, wasm32-unknown-unknown)	30s
minimal-versions / minimal-versions	1m 10s
	5m 58s

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