[Ada] Don't check for misspelling of Not_A_Restriction_Id

When looking for a misspelling of a restriction identifier we should
ignore the Not_A_Restriction_Id literal, because it doesn't represent
any restriction.

gcc/ada/

	* sem_prag.adb (Process_Restrictions_Or_Restriction_Warnings):
	Fix range of iteration.

gcc/ada/sem_prag.adb

@@ -10561,7 +10561,7 @@ package body Sem_Prag is
 
                   --  Check for possible misspelling
 
-                  for J in Restriction_Id loop
+                  for J in All_Restrictions loop
                      declare
                         Rnm : constant String := Restriction_Id'Image (J);