Fix compiler warnings on Windows introduced in bpo-13617. (#2464)

Modules/_ctypes/callproc.c

@@ -1249,7 +1249,7 @@ The handle may be used to locate exported functions in this\n\
 module.\n";
 static PyObject *load_library(PyObject *self, PyObject *args)
 {
-    WCHAR *name;
+    const WCHAR *name;
     PyObject *nameobj;
     PyObject *ignored;
     HMODULE hMod;

PC/_msi.c

@@ -594,7 +594,7 @@ summary_setproperty(msiobj* si, PyObject *args)
         return NULL;
 
     if (PyUnicode_Check(data)) {
-        WCHAR *value = _PyUnicode_AsUnicode(data);
+        const WCHAR *value = _PyUnicode_AsUnicode(data);
         if (value == NULL) {
             return NULL;
         }