As seen in include/sfx2/dispatch.hxx IWYU proposes to replace
o3tl/span.hxx with fw declaration, but that won't compile

Change-Id: If5739075bd91511cf22a39f3382c424c21829053
Reviewed-on: https://gerrit.libreoffice.org/69397
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>