gdb: constify inferior::target_is_pushed

Change-Id: Ia4143b9c63cb76e2c824ba773c66f5c5cd94b2aa
diff --git a/gdb/inferior.h b/gdb/inferior.h
index bb3aec1..2776530 100644
--- a/gdb/inferior.h
+++ b/gdb/inferior.h
@@ -369,7 +369,7 @@ class inferior : public refcounted_object,
   int unpush_target (struct target_ops *t);
 
   /* Returns true if T is pushed in this inferior's target stack.  */
-  bool target_is_pushed (const target_ops *t)
+  bool target_is_pushed (const target_ops *t) const
   { return m_target_stack.is_pushed (t); }
 
   /* Find the target beneath T in this inferior's target stack.  */