Blind attempt to fix #13626

The error message was:

```
error: unknown character escape: d
  --> C:\projects\servo\target\debug\build\style-9b556a6c7bb4f044\out/properties.rs:44:55
   |
44 | #[path="C:/projects/servo/components/style/properties\declaration_block.rs"]
   |
```

Use `repr()` to get a Python string literal (whose character escaping
is compatible enough with Rust’s) and `[1:-1]` to remove the single quotes.

components/style/properties/properties.mako.rs

@@ -44,7 +44,7 @@ pub use self::declaration_block::*;
     import os.path
 %>
 
-#[path="${os.path.join(os.path.dirname(__file__), 'declaration_block.rs')}"]
+#[path="${repr(os.path.join(os.path.dirname(__file__), 'declaration_block.rs'))[1:-1]}"]
 pub mod declaration_block;
 
 pub mod longhands {