der_derive: add workaround for `syn` v2.0.73 #2077

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	2m 24s
build (1.65.0, wasm32-unknown-unknown)	2m 10s
test (1.65.0)	2m 17s
build (stable, thumbv7em-none-eabi)	17s
test (stable)	14s
build (stable, wasm32-unknown-unknown)	18s
minimal-versions / minimal-versions	34s
	8m 14s

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