x86/build/vdso: fix missing FORCE for *.so build rule

Add FORCE so that if_changed can detect the command line change.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

arch/x86/entry/vdso/Makefile

@@ -131,7 +131,7 @@ $(obj)/%-x32.o: $(obj)/%.o FORCE
 targets += vdsox32.lds $(vobjx32s-y)
 
 $(obj)/%.so: OBJCOPYFLAGS := -S --remove-section __ex_table
-$(obj)/%.so: $(obj)/%.so.dbg
+$(obj)/%.so: $(obj)/%.so.dbg FORCE
 	$(call if_changed,objcopy)
 
 $(obj)/vdsox32.so.dbg: $(obj)/vdsox32.lds $(vobjx32s) FORCE