Fixup deprecated API usages from prost upgrade

Summary:
Replace deprecate `from_i32` API from older version of prost.

Needed to bump buck2 Cargo deps to make OSS build happy

Reviewed By: dtolnay

Differential Revision: D69620522

fbshipit-source-id: 5835ff1451358617d47f796075a3e8cdd676434e

Author: capickett

allocative/allocative/Cargo.toml

@@ -24,7 +24,7 @@ indexmap = { version = "2.2.6", optional = true }
 num-bigint = { version = "0.4.3", optional = true }
 once_cell = { version = "1.15.0", optional = true }
 parking_lot = { version = "0.11.2", optional = true }
-prost-types = { version = "0.11.2", optional = true }
+prost-types = { version = "0.12.3", optional = true }
 relative-path = { version = "1.7.0", optional = true }
 serde_json = { version = "1.0.48", optional = true }
 slab = { version = "0.4.7", optional = true }