kbuild: simplify generic vdso installation code

With commit 4b0bf9a0 ("riscv: compat_vdso: install compat_vdso.so.dbg
to /lib/modules/*/vdso/") applied, all debug VDSO files are installed in
$(MODLIB)/vdso/.

Simplify the installation rule.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>