sepolicy: Fix legacy build variable

BOARD_SEPOLICY_DIRS is deprecated.

See system/sepolicy/Android.mk:
> # BOARD_SEPOLICY_DIRS was used for vendor/odm sepolicy customization
> # before. It has been replaced by BOARD_VENDOR_SEPOLICY_DIRS (mandatory)
> # and BOARD_ODM_SEPOLICY_DIRS (optional). BOARD_SEPOLICY_DIRS is still
> # allowed for backward compatibility, which will be merged into
> # BOARD_VENDOR_SEPOLICY_DIRS.
> ifdef BOARD_SEPOLICY_DIRS
> BOARD_VENDOR_SEPOLICY_DIRS += $(BOARD_SEPOLICY_DIRS)
> endif

Change-Id: Ic1a49395af356130f351445f898a5cd29512f20f
diff --git a/sepolicy/sepolicy.mk b/sepolicy/sepolicy.mk
index f4bab81..3759983 100644
--- a/sepolicy/sepolicy.mk
+++ b/sepolicy/sepolicy.mk
@@ -3,7 +3,7 @@
 # inherit from Omni
 #
 
-BOARD_SEPOLICY_DIRS += \
+BOARD_VENDOR_SEPOLICY_DIRS += \
     vendor/omni/sepolicy/common
 
 BOARD_PLAT_PRIVATE_SEPOLICY_DIR += vendor/omni/sepolicy/private