fix compilation errors if only the feature hash2curve is enabled #1952

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	4m 33s
build (1.65.0, wasm32-unknown-unknown)	4m 33s
build (stable, thumbv7em-none-eabi)	35s
doc	24s
build (stable, wasm32-unknown-unknown)	39s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	5m 42s
cross (armv7-unknown-linux-gnueabihf, 1.65.0)	5m 27s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 44s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 36s
test (x86_64-unknown-linux-gnu, 1.65.0)	5m 21s
cross (aarch64-unknown-linux-gnu, 1.65.0)	5m 54s
test (x86_64-unknown-linux-gnu, stable)	1m 25s
cross (aarch64-unknown-linux-gnu, stable)	1m 27s
cross (powerpc-unknown-linux-gnu, 1.65.0)	5m 28s
cross (powerpc-unknown-linux-gnu, stable)	1m 27s
	46m 15s

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