[PATCH] kconfig: add "void conf_set_changed_callback(void (*fn)(void))", use it in qconf.cc

Added function sets "void (*conf_changed_callback)(void)".  Call it, if
.config's changed state changes.  Use above in qconf.cc to set gui's
save-widget's sensitvity.

Signed-off-by: Karsten Wiese <fzu@wemgehoertderstaat.de>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Roman Zippel <zippel@linux-m68k.org>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>