chore(deps): update external/libexpat digest to b1ab474 #12

Job	Run time
lib_linux (monero)	2m 31s
lib_android (monero)	26m 3s
lib_mingw (monero)	3m 21s
lib_linux (wownero)	2m 45s
lib_android (wownero)	24m 17s
lib_mingw (wownero)	3m 29s
lib_sailfishos_aarch64 (monero)	3m 46s
lib_sailfishos_aarch64 (wownero)	4m 16s
lib_darwin (monero)	13m 28s
lib_darwin (wownero)	13m 25s
ios build (monero)	12m 40s
ios build (wownero)	13m 2s
macos build (monero)	3m 29s
macos build (wownero)	3m 38s
lib_sailfishos_i486 (monero)	2m 4s
lib_sailfishos_i486 (wownero)	2m 4s
wallet_cake_android	20m 42s
wallet_xmruw_android (monero)	6m 47s
wallet_xmruw_android (wownero)	6m 52s
comment on pr	4s
	2h 48m 43s

Provide feedback