find-unneeded-includes: don't try to fw. declare o3tl/span.hxx

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>

bin/find-unneeded-includes

@@ -70,6 +70,7 @@ def ignoreRemoval(include, toAdd, absFileName, moduleRules):
     o3tl = {
         "o3tl/typed_flags_set.hxx" : "namespace o3tl { template <typename T> struct typed_flags; }",
         "o3tl/deleter.hxx" : "namespace o3tl { template <typename T> struct default_delete; }",
+        "o3tl/span.hxx" : "namespace o3tl { template <typename T> class span; }",
     }
     for k, v, in o3tl.items():
         if include == k and v in toAdd: