[mlir][python] handle more undefined symbols not covered by nanobind (#153861)

Introduced (but omitted from this CMake) in
llvm/llvm-project#151246.

Author: makslevental

mlir/lib/Bindings/Python/IRCore.cpp

@@ -2810,7 +2810,7 @@ class PyOpAttributeMap {
 
 // bpo-42262 added Py_XNewRef()
 #if !defined(Py_XNewRef)
-PyObject *_Py_XNewRef(PyObject *obj) {
+[[maybe_unused]] PyObject *_Py_XNewRef(PyObject *obj) {
   Py_XINCREF(obj);
   return obj;
 }
@@ -2819,7 +2819,7 @@ PyObject *_Py_XNewRef(PyObject *obj) {
 
 // bpo-42262 added Py_NewRef()
 #if !defined(Py_NewRef)
-PyObject *_Py_NewRef(PyObject *obj) {
+[[maybe_unused]] PyObject *_Py_NewRef(PyObject *obj) {
   Py_INCREF(obj);
   return obj;
 }