Update bitfield-struct version (#111)

Updated bitfield-struct dependency version from 0.10 to 0.12. Shouldn't
need any actual code changes.

Author: smalis-msft

Cargo.toml

@@ -13,7 +13,7 @@ igvm_defs = { path = "igvm_defs", version = "0.4.0" }
 igvm = { path = "igvm", version = "0.4.0" }
 
 anyhow = "1.0"
-bitfield-struct = "0.10"
+bitfield-struct = "0.12"
 crc32fast = { version = "1.3.2", default-features = false }
 hex = { version = "0.4", default-features = false }
 open-enum = "0.5.2"