bpo-29565: Fix compilation for C89 (GH-8626)

Use a local scope for the 'i' variable.

Modules/_ctypes/libffi_msvc/ffi.c

@@ -220,9 +220,12 @@ ffi_call(/*@dependent@*/ ffi_cif *cif,
       break;
 #else
     case FFI_SYSV:
+      /* use a local scope for the 'i' variable */
+      {
+          unsigned i;
           /* If a single argument takes more than 8 bytes,
              then a copy is passed by reference. */
-      for (unsigned i = 0; i < cif->nargs; i++) {
+          for (i = 0; i < cif->nargs; i++) {
               size_t z = cif->arg_types[i]->size;
               if (z > 8) {
                   void *temp = alloca(z);
@@ -230,6 +233,7 @@ ffi_call(/*@dependent@*/ ffi_cif *cif,
                   avalue[i] = temp;
               }
           }
+      }
       /*@-usedef@*/
       return ffi_call_AMD64(ffi_prep_args, &ecif, cif->bytes,
 			   cif->flags, ecif.rvalue, fn);