Fix building with gcc-4.7

https://bugs.gentoo.org/show_bug.cgi?id=419165
signed-of-by: Joerg Bornkessel <hd_brummy@gentoo.org> 2012/06/10

diff -Naur vdr-1.6.0.orig/config.h vdr-1.6.0/config.h
--- vdr-1.6.0.orig/config.h	2012-06-10 17:41:26.000000000 +0200
+++ vdr-1.6.0/config.h	2012-06-10 17:42:19.000000000 +0200
@@ -182,7 +182,7 @@
 #ifdef USE_CMDSUBMENU
                       AddConfig(l);
 #else
-                      Add(l);
+                      this->Add(l);
 #endif /* CMDSUBMENU */
                    else {
                       esyslog("ERROR: error in %s, line %d", fileName, line);