Makefile: add qga-vss-dll-obj-y to nested variables

The build rule for qga/vss-win32/qga-vss.dll is broken by commit
ba1183da, because it misses
qga-vss-dll-obj-y in the list of nested variables.
This fixes build of qga-vss.dll by adding qga-vss-dll-obj-y to the list.
Signed-off-by: NTomoki Sekiyama <tomoki.sekiyama@hds.com>
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

Makefile

@@ -133,6 +133,7 @@ dummy := $(call unnest-vars,, \
                 stub-obj-y \
                 util-obj-y \
                 qga-obj-y \
+                qga-vss-dll-obj-y \
                 block-obj-y \
                 block-obj-m \
                 common-obj-y \