Fix a conflict between fmt::join and fmt/ostream.h (#744)

test/ostream-test.cc

@@ -158,3 +158,8 @@ TEST(OStreamTest, WriteToOStreamMaxSize) {
   } while (size != 0);
   fmt::internal::write(os, buffer);
 }
+
+TEST(OStreamTest, Join) {
+  int v[3] = {1, 2, 3};
+  EXPECT_EQ("1, 2, 3", fmt::format("{}", fmt::join(v, v + 3, ", ")));
+}