[PATCH] Makefile: fix for compatibility with *emacs ctags

I've noticed that, starting from linux-2.6.12-rc1, in the top Makefile the
"cmd_tags" variable has been changed in a way incompatible with *emacs
ctags.  Since the "--extra" option exists only in "exuberant ctags", it
should be included in the CTAGSF shell variable.

Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>