in rustc_index: fix compilation error by changing the rustc_serialize dependency to be required instead of optional

Author: Tage Johansson

compiler/rustc_index/Cargo.toml

@@ -8,15 +8,14 @@ edition = "2024"
 itertools = "0.12"
 rustc_index_macros = { path = "../rustc_index_macros" }
 rustc_macros = { path = "../rustc_macros", optional = true }
-rustc_serialize = { path = "../rustc_serialize", optional = true }
+rustc_serialize = { path = "../rustc_serialize" }
 smallvec = "1.8.1"
 # tidy-alphabetical-end
 
 [features]
 # tidy-alphabetical-start
 default = ["nightly"]
 nightly = [
-    "dep:rustc_serialize",
     "dep:rustc_macros",
     "rustc_index_macros/nightly",
 ]