powerpc/vdso: Make sure vdso_wrapper.o is rebuilt everytime vdso.so is rebuilt

Commit bce74491 ("powerpc/vdso: fix unnecessary rebuilds of
vgettimeofday.o") moved vdso32_wrapper.o and vdso64_wrapper.o out
of arch/powerpc/kernel/vdso[32/64]/ and removed the dependencies in
the Makefile. This leads to the wrappers not being re-build hence the
kernel embedding the old vdso library.

Add back missing dependencies to ensure vdso32_wrapper.o and vdso64_wrapper.o
are rebuilt when vdso32.so.dbg and vdso64.so.dbg are changed.

Fixes: bce74491 ("powerpc/vdso: fix unnecessary rebuilds of vgettimeofday.o")
Cc: stable@vger.kernel.org
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/8bb015bc98c51d8ced581415b7e3d157e18da7c9.1617181918.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/Makefile

@@ -191,3 +191,7 @@ $(obj)/prom_init_check: $(src)/prom_init_check.sh $(obj)/prom_init.o FORCE
 targets += prom_init_check
 
 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