fix include type src failure

Summary:
before this diff
```
cd ~/fbsource/fbcode/common/rust/cargo_from_buck/fbcode/security/flare/if/flare_if
cargo build
```
fails with errors (P1172791642)
```
Failed while running thrift compilation: No such file or directory (os error 2)
```

Reviewed By: dtolnay

Differential Revision:
D53382391

Privacy Context Container: L1124100

fbshipit-source-id: fdc9d541fb75c53b1aba3da1f57976846de78f9f

shed/thrift_compiler/src/lib.rs

@@ -197,11 +197,15 @@ impl Config {
         }
         for lib_include_src in &self.lib_include_srcs {
             println!("cargo:rerun-if-changed={lib_include_src}");
-            fs::copy(lib_include_src, out.join(lib_include_src))?;
+            if let GenContext::Lib = self.gen_context {
+                fs::copy(lib_include_src, out.join(lib_include_src))?;
+            }
         }
         for types_include_src in &self.types_include_srcs {
             println!("cargo:rerun-if-changed={types_include_src}");
-            fs::copy(types_include_src, out.join(types_include_src))?;
+            if let GenContext::Types = self.gen_context {
+                fs::copy(types_include_src, out.join(types_include_src))?;
+            }
         }
 
         if let [(_name, file)] = &input[..] {