Put std feature of tz-rs behind the std feature #664

Job	Run time
cross-miri (aarch64-unknown-linux-gnu)	37s
cross-miri (i586-unknown-linux-gnu)	38s
clippy-pedantic (tzdb)	15s
cross-miri (powerpc-unknown-linux-gnu)	35s
DevSkim	33s
clippy-pedantic (tzdb_data)	13s
audit	17s
cross-miri (powerpc64-unknown-linux-gnu)	39s
cross-miri (powerpc64le-unknown-linux-gnu)	35s
doc	17s
fmt	15s
lint (1.81)	16s
feature-powerset	15s
Miri	48s
lint (1.81, -Zminimal-versions)	18s
test	30s
lint (stable)	18s
typos	5s
lint (stable, -Zminimal-versions)	12s
lint (nightly)	21s
lint (nightly, -Zminimal-versions)	21s
	8m 18s

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