[fltk] Update fltk to version 1.3.7

[MoAlyousef]

Describe the pull request

• What does your PR fix?

Update fltk port to version 1.3.7

• Which triplets are supported/not supported? Have you updated the [CI baseline]

All triplets except uwp.

• Does your PR follow the maintainer guide?

Yes

• If you have added/updated a port: Have you run ./vcpkg x-add-version --all and committed the result?

Yes

If you are still working on the PR, open it as a Draft: https://github.blog/2019-02-14-introducing-draft-pull-requests/

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After committing all other changes, the version database must be updated

git add -u && git commit
git checkout 6bc4362fb49e53f1fff7f51e4e27e1946755ecc6 -- versions
./vcpkg x-add-version --all

Diff

diff --git a/versions/f-/fltk.json b/versions/f-/fltk.json
index f09d3f1..4786b8b 100644
--- a/versions/f-/fltk.json
+++ b/versions/f-/fltk.json
@@ -1,7 +1,7 @@
 {
   "versions": [
     {
-      "git-tree": "76c5cdaba5b3b8edf2230cc36c6677f64a2e372e",
+      "git-tree": "3fff1ed0fe94c6d45ad15d6cb6f575537880f1f6",
       "version": "1.3.7",
       "port-version": 0
     },