add '--extra=+f' to ctags in Makefile in order to search for file names

I usually add the following to ctags in the Makefile so I can search for file names in my editor. If you think others would find this useful too, then please apply.

@@ -1167,7 +1167,7 @@
 define cmd_tags
        rm -f $@; \
        CTAGSF=`ctags --version | grep -i exuberant >/dev/null && echo

"-I __initdata,__exitdata,EXPORT_SYMBOL,EXPORT_SYMBOL_NOVERS"`; \

-       $(all-sources) | xargs ctags $$CTAGSF -a
+       $(all-sources) | xargs ctags $$CTAGSF -a --extra=+f

endef

TAGS: FORCE