Fix "enumeration values not handled in switch" error in testsuite

When running the testsuite with clang, gdb.base/sigaltstack.c
fails to compile with the following error:
  warning: enumeration values 'LEAF' and 'NR_LEVELS' not handled
    in switch [-Wswitch]

This prevents the gdb.base/sigaltstack.exp from executing.
This commit fixes.

gdb/testsuite/ChangeLog:

	* gdb.base/sigaltstack.c (catcher): Add default case to switch
	statement.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2020-05-28  Gary Benson <gbenson@redhat.com>
+
+	* gdb.base/sigaltstack.c (catcher): Add default case to switch
+	statement.
+
 2020-05-28  Gary Benson <gbenson@redhat.com>
 
 	* gdb.cp/classes.exp (prepare_for_testing): Add

gdb/testsuite/gdb.base/sigaltstack.c

@@ -68,6 +68,8 @@ catcher (int signal)
     case INNER:
       level = LEAF;
       return;
+    default:
+      abort ();
     }
 }