kbuild: fix tags generation of config symbols

commit 4f628248

 aka "kbuild: reintroduce
ALLSOURCE_ARCHS support for tags/cscope" breaks tags generation for
Kconfig symbols.

Steps to reproduce:

	make tags
	vi -t PROC_FS

It should jump to 'config PROC_FS' line.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Tested-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>