[opam] add missing dependency

dune-project

@@ -35,6 +35,7 @@
   "camlp-streams"
   ("conf-llvm" (>= 11.0.0)) ; note: includes depexts
   ("conf-libclang" (>= 11.0.0))
+   "conf-clang"
      ; conf-libclang-cpp:version should be equal to conf-llvm:version but
      ; this can't be enforced through opam dependencies yet
   "conf-cmake" ; note: includes depexts

frama-clang.opam

@@ -9,6 +9,7 @@ depends: [
   "camlp-streams"
   "conf-llvm" {>= "11.0.0"}
   "conf-libclang" {>= "11.0.0"}
+  "conf-clang"
   "conf-cmake"
   "odoc" {with-doc}
 ]