secp256k1-tr: fix no-std support #1398

# to view logs

Job	Run time
build with default features	29s
build with no_std (ristretto255)	18s
build with MSRV (1.66.1)	2m 36s
build with no_std (ed25519)	22s
actionlint	7s
build with no_std (p256)	32s
Clippy	31s
build with no_std (secp256k1)	23s
Check Rust doc	29s
Rustfmt	17s
build with no_std (secp256k1-tr)	21s
build with no_std (rerandomized)	28s
Check if automatically generated code is up to date	19s
test on beta	2m 26s
	9m 38s

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