Fix build dependency error in spellcheck mojo

spellcheck/common:interfaces (via typemap) uses some
spellcheck_result.h and spellcheck_struct_traits.h so those
cannot be in a target "above" the mojom build target.

Bug: 938893
Change-Id: Ic3a307248542dfa9b02cb7fb548d88734c6db509
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1528192
Auto-Submit: Daniel Bratell <bratell@opera.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#643831}