sparc: vdso: clean up build artifacts in arch/sparc/vdso/

Currently, vdso-image-*.c, vdso*.so, vdso*.so.dbg are not cleaned
because 'make clean' does not include include/config/auto.conf,
resulting in $(vdso_img-y) being empty.

Add the build artifacts to 'targets' unconditionally.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>