protols: fix well-known types/imports support (#465302)

pkgs/by-name/pr/protols/package.nix

@@ -1,5 +1,6 @@
 {
   lib,
+  protobuf,
   rustPlatform,
   fetchFromGitHub,
 }:
@@ -17,6 +18,8 @@ rustPlatform.buildRustPackage rec {
 
   cargoHash = "sha256-q3GMizdBupQSMVCuRqLjuw0Mof1q3UYOdUBugmrTDMU=";
 
+  env.FALLBACK_INCLUDE_PATH = "${protobuf}/include";
+
   meta = {
     description = "Protocol Buffers language server written in Rust";
     homepage = "https://github.com/coder3101/protols";