vendor: bring back sepolicy's union directive

fix the build process
http://paste.omnirom.org/view/54d5e5c1

Signed-off-by: Humberto Borba <humberos@gmail.com>
Change-Id: Ieacfa4dcea7634c49db869970d1fa97194febafd
diff --git a/sepolicy/sepolicy.mk b/sepolicy/sepolicy.mk
index 9b9c568..b78666d 100644
--- a/sepolicy/sepolicy.mk
+++ b/sepolicy/sepolicy.mk
@@ -6,3 +6,10 @@
 BOARD_SEPOLICY_DIRS += \
     vendor/omni/sepolicy
 
+BOARD_SEPOLICY_UNION += \
+    file_contexts \
+    file.te \
+    genfs_contexts \
+    system_app.te \
+    system_server.te \
+    vold.te