Do not require the `thiserror` to be explicitly imported.

[drahnr]

error[E0433]: failed to resolve: use of undeclared crate or module `thiserror`
  --> node/network/statement-distribution/src/error.rs:38:1
   |
38 | #[fatality::fatality(splitable)]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ use of undeclared crate or module `thiserror`
   |
   = note: this error originates in the derive macro `::fatality::thiserror::Error` (in Nightly builds, run with -Z macro-backtrace for more info)

   Compiling polkadot-rpc v0.9.17 (/media/supersonic1t/projects/parity/polkadot-bernhard-fatality/rpc)
   Compiling pallet-treasury v4.0.0-dev (https://github.com/paritytech/substrate?branch=master#42b2d623)
   Compiling pallet-offences v4.0.0-dev (https://github.com/paritytech/substrate?branch=master#42b2d623)
error[E0599]: no method named `as_dyn_error` found for reference `&SubsystemError` in the current scope
  --> node/network/statement-distribution/src/error.rs:49:10
   |
49 |     #[error("Spawning subsystem task failed")]
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ method not found in `&SubsystemError`
   |
   = help: items from traits can only be used if the trait is in scope
help: the following trait is implemented but not in scope; perhaps add a `use` for it:
   |
20 | use fatality::thiserror::private::AsDynError;
   |

error[E0599]: no method named `as_dyn_error` found for reference `&SubsystemError` in the current scope
  --> node/network/statement-distribution/src/error.rs:53:10
   |
53 |     #[error("Receiving message from overseer failed")]
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ method not found in `&SubsystemError`
   |
   = help: items from traits can only be used if the trait is in scope
help: the following trait is implemented but not in scope; perhaps add a `use` for it:
   |
20 | use fatality::thiserror::private::AsDynError;
   |

error[E0599]: no method named `as_dyn_error` found for reference `&polkadot_node_subsystem_util::runtime::Error` in the current scope
  --> node/network/statement-distribution/src/error.rs:57:10
   |
57 |     #[error("Error while accessing runtime information")]
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ method not found in `&polkadot_node_subsystem_util::runtime::Error`
   |
   = help: items from traits can only be used if the trait is in scope
help: the following trait is implemented but not in scope; perhaps add a `use` for it:
   |
20 | use fatality::thiserror::private::AsDynError;
   |

Some errors have detailed explanations: E0433, E0599.
For more information about an error, try `rustc --explain E0433`.
error: could not compile `polkadot-statement-distribution` due to 6 previous errors
warning: build failed, waiting for other jobs to finish...
error: build failed