Added `explicit` to a single-arg constructor to fix a ClangTidy warning.

This is the only remaining ClangTidy warning in this directory.

PiperOrigin-RevId: 593867029

upb/wire/eps_copy_input_stream_test.cc

diff --git a/upb/wire/eps_copy_input_stream_test.cc b/upb/wire/eps_copy_input_stream_test.cc
index 7a3caa2..c28d145 100644
--- a/upb/wire/eps_copy_input_stream_test.cc
+++ b/upb/wire/eps_copy_input_stream_test.cc
@@ -34,7 +34,7 @@
 //
 // class FakeStream {
 //  public:
-//   FakeStream(const std::string& data) : data_(data), offset_(0) {
+//   explicit FakeStream(const std::string& data) : data_(data), offset_(0) {
 //     limits_.push_back(data.size());
 //   }
 //