powerpc: clean vdso32 and vdso64 directories

Since commit bce7449 ("powerpc/vdso: fix unnecessary rebuilds of
vgettimeofday.o"), "make ARCH=powerpc clean" does not clean up the
arch/powerpc/kernel/{vdso32,vdso64} directories.

Use the subdir- trick to let "make clean" descend into them.

Fixes: bce7449 ("powerpc/vdso: fix unnecessary rebuilds of vgettimeofday.o")
Signed-off-by: Masahiro Yamada <[email protected]>
Signed-off-by: Michael Ellerman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: masahir0y

arch/powerpc/kernel/Makefile

@@ -196,3 +196,6 @@ clean-files := vmlinux.lds
 # Force dependency (incbin is bad)
 $(obj)/vdso32_wrapper.o : $(obj)/vdso32/vdso32.so.dbg
 $(obj)/vdso64_wrapper.o : $(obj)/vdso64/vdso64.so.dbg
+
+# for cleaning
+subdir- += vdso32 vdso64