Fixed a preprocessor guard in pybind11_absl.h

tensorflow::StringPiece is an alias to std::string_view only if
ABSL_USES_STD_STRING_VIEW is defined, otherwise we do need to register
a type caster for it.
PiperOrigin-RevId: 281329783
Change-Id: I34ec3ed52496d467666670cf691bebf18f30b3b1

tensorflow/python/lib/core/pybind11_absl.h

@@ -19,7 +19,7 @@ limitations under the License.
 #include "pybind11/pybind11.h"
 #include "tensorflow/core/platform/stringpiece.h"
 
-#if !defined(PYBIND11_CPP17)
+#ifndef ABSL_USES_STD_STRING_VIEW
 
 namespace pybind11 {
 namespace detail {
@@ -36,5 +36,5 @@ struct type_caster<tensorflow::StringPiece>
 }  // namespace detail
 }  // namespace pybind11
 
-#endif  // !defined(PYBIND11_CPP17)
+#endif  // ABSL_USES_STD_STRING_VIEW
 #endif  // TENSORFLOW_PYTHON_LIB_CORE_PYBIND11_ABSL_H_