server: fix cargo build warning

[bergwolf]

clippy doesn't like the solution proposed by rustc. Let's just use the plain if let statement.

error: unused return value of std::option::Option::<T>::map_or that must be used --> src/api/server/sync_io.rs:93:9	93	hook.map_or((),	h	h.collect(&in_header));	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

= note: if you don't need the returned value, use if let instead = note: -D unused-must-use implied by -D warnings