fix build: deptopt on mtime for trace-core.util

dune-project

@@ -18,6 +18,8 @@
  (depends
    (ocaml (>= 4.08))
    dune)
+ (depopts
+   (mtime (>= 2.0)))
  (tags
   (trace tracing observability profiling)))
 

src/util/dune

@@ -3,6 +3,7 @@
   (public_name trace.private.util)
   (synopsis "internal utilities for trace. No guarantees of stability.")
   (name trace_private_util)
+  (optional) ; depends on mtime
   (libraries trace.core mtime mtime.clock.os unix threads
              (select domain_util.ml from
                      (base-domain -> domain_util.real.ml)

trace.opam

@@ -14,6 +14,9 @@ depends: [
   "dune" {>= "2.9"}
   "odoc" {with-doc}
 ]
+depopts: [
+  "mtime" {>= "2.0"}
+]
 build: [
   ["dune" "subst"] {dev}
   [