[opam] backport missing dependency

as mentioned by kit-ty-kate on github, if we are to launch autoonf, we should depend on conf-autoconf

opam

@@ -38,6 +38,7 @@ depends: [
   "ocaml" { >= "4.08.1" }
   "frama-c" { >= "25.0" & < "26.0" }
   "why3" { >= "1.5.0" }
+  "conf-autoconf" {dev}
 ]
 
 depopts: [ "conf-swi-prolog" ]