BUG: generate_numpy_api: fix C90 violation

Encountered this because of gcc-14.
ISO C90 forbids mixed declarations and code

Author: arachnist

numpy/_core/code_generators/generate_numpy_api.py

@@ -65,6 +65,7 @@
 {
   int st;
   PyObject *numpy = PyImport_ImportModule("numpy._core._multiarray_umath");
+  PyObject *c_api;
   if (numpy == NULL && PyErr_ExceptionMatches(PyExc_ModuleNotFoundError)) {
     PyErr_Clear();
     numpy = PyImport_ImportModule("numpy.core._multiarray_umath");
@@ -74,7 +75,7 @@
       return -1;
   }
 
-  PyObject *c_api = PyObject_GetAttrString(numpy, "_ARRAY_API");
+  c_api = PyObject_GetAttrString(numpy, "_ARRAY_API");
   Py_DECREF(numpy);
   if (c_api == NULL) {
       return -1;