Add missing 'requires coroutines' to module map

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@304180 91177308-0d34-0410-b5e6-96231b3b80d8

include/module.modulemap

@@ -502,6 +502,7 @@ module std [system] {
       export *
     }
      module coroutine {
+      requires coroutines
       header "experimental/coroutine"
       export *
     }