no_mangle warning when compiling with 'network' feature

[frewsxcv]

warning: attribute should be applied to a function or static
  --> src/network.rs:28:1
   |
28 |   #[no_mangle]
   |   ^^^^^^^^^^^^
29 | / struct HandleData {
30 | |     request: reqwest::blocking::RequestBuilder,
31 | |     headers: reqwest::header::HeaderMap,
32 | |     // this raw pointer is returned to libproj but never returned from libproj,
...  |
37 | |     hptr: Option<*const c_char>,
38 | | }
   | |_- not a function or static
   |
   = note: `#[warn(unused_attributes)]` on by default
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!

[urschrei]

Already in progress in #59!