der_derive: add workaround for `syn` v2.0.73 #2469

Job	Run time
build (1.71.0, thumbv7em-none-eabi)	19s
test (1.71.0)	22s
build (1.71.0, wasm32-unknown-unknown)	16s
test (stable)	18s
build (stable, thumbv7em-none-eabi)	18s
build (stable, wasm32-unknown-unknown)	18s
minimal-versions / minimal-versions	33s
	2m 24s

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