Add `PYBIND11_WARNING_DISABLE_CLANG("-Wgnu-zero-variadic-macro-argume…

…nts")` in test_smart_ptr.cpp

This is the error message:

```
/__w/pybind11/pybind11/tests/test_smart_ptr.cpp:287:51: error: must specify at least one argument for '...' parameter of variadic macro [-Werror,-Wgnu-zero-variadic-macro-arguments]
PYBIND11_DECLARE_HOLDER_TYPE(T, std::shared_ptr<T>)
                                                  ^
/__w/pybind11/pybind11/include/pybind11/cast.h:885:13: note: macro 'PYBIND11_DECLARE_HOLDER_TYPE' defined here
            ^
```

tests/test_smart_ptr.cpp

@@ -11,8 +11,9 @@
 #include "object.h"
 #include "pybind11_tests.h"
 
-// This breaks on PYBIND11_DECLARE_HOLDER_TYPE
+// These break on PYBIND11_DECLARE_HOLDER_TYPE
 PYBIND11_WARNING_DISABLE_GCC("-Wpedantic")
+PYBIND11_WARNING_DISABLE_CLANG("-Wgnu-zero-variadic-macro-arguments")
 
 namespace {