pgrep: fix uucore feature dependencies #1692

Job	Run time
cargo test (windows-latest)	3m 20s
cargo test (ubuntu-latest)	1m 17s
cargo check (windows-latest)	1m 56s
cargo check (macOS-latest)	25s
Code Coverage (ubuntu-latest, unix)	3m 17s
Code Coverage (macos-latest, macos)	1m 56s
cargo test (macOS-latest)	1m 3s
cargo check (ubuntu-latest)	1m 57s
Code Coverage (windows-latest, windows)	4m 33s
	19m 44s

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