fat-filesystem requires result

Fails with
```
=== ERROR while compiling fat-filesystem.0.13.0 ==============================#
 context              2.2.0~alpha3 | linux/x86_64 | ocaml-base-compiler.4.14.1 | file:///home/opam/opam-repository
 path                 ~/.opam/4.14/.opam-switch/build/fat-filesystem.0.13.0
 command              ~/.opam/opam-init/hooks/sandbox.sh build dune build -p fat-filesystem -j 71
 exit-code            1
 env-file             ~/.opam/log/fat-filesystem-7-176cf6.env
 output-file          ~/.opam/log/fat-filesystem-7-176cf6.out
 File "src/dune", line 5, characters 3-9:
 5 |    result rresult astring)
        ^^^^^^
 Error: Library "result" not found.
 -> required by library "fat-filesystem" in _build/default/src
 -> required by _build/default/META.fat-filesystem
 -> required by _build/install/default/lib/fat-filesystem/META
 -> required by _build/default/fat-filesystem.install
 -> required by alias install
```
Signed-off-by: Marcello Seri <[email protected]>

packages/fat-filesystem/fat-filesystem.0.13.0/opam

@@ -18,6 +18,7 @@ depends: [
   "mirage-block-unix" {>= "2.5.0"}
   "io-page-unix" {>= "2.0.0"}
   "re" {>= "1.7.2"}
+  "result"
   "cmdliner"
   "astring"
   "alcotest" {with-test}