Fix compilation errors with some feature combinations

[FreezyLemon]

v_frame is only included when diff and/or estimate is enabled:

av1-grain/Cargo.toml

Lines 26 to 33 in f0292b0

	[features]
	default = ["create", "parse", "diff", "estimate"]
	unstable = []
	create = []
	diff = ["num-rational", "v_frame"]
	estimate = ["v_frame"]
	parse = ["nom"]
	serialize = ["serde", "arrayvec/serde"]

src/util.rs is only used with diff enabled at the moment.

I checked with cargo hack --feature-powerset check that this compiles

[coveralls]

Pull Request Test Coverage Report for Build 8501393001

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.001%) to 26.517%

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Totals
Change from base Build 7409066287:	-0.001%
Covered Lines:	555
Relevant Lines:	2093

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💛 - Coveralls

[lu-zero]

Thank you