fix(bdk_electrum): build with `--no-default-features` #110

Job	Run time
Check no_std	3m 7s
Build Examples (example_cli)	2m 41s
Build Examples (example_bitcoind_rpc_polling)	32s
Build and test (stable, true, --no-default-features)	4m 9s
Check WASM	3m 31s
Build Examples (example_electrum)	49s
Build and test (stable, true, --all-features)	6m 16s
Rust fmt	8s
Build Examples (example_esplora)	52s
Build and test (1.63.0, --no-default-features)	7m 23s
Build Examples (wallet_electrum)	54s
Build and test (1.63.0, --all-features)	5m 1s
prepare	2s
Build Examples (wallet_esplora_async)	59s
Build Examples (wallet_esplora_blocking)	50s
Build Examples (wallet_rpc)	33s
clippy_check	3m 34s
	41m 21s

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