omni: use TARGET_COPY_OUT_SYSTEM instead of system/

Change-Id: Iedd3044e0e747a9b8ffe2cb4c8648035655af3aa
diff --git a/config/gsm.mk b/config/gsm.mk
index 17ddbca..cf12e83 100644
--- a/config/gsm.mk
+++ b/config/gsm.mk
@@ -1,6 +1,6 @@
 # GSM APN list
 PRODUCT_COPY_FILES += \
-    vendor/omni/prebuilt/etc/apns-conf.xml:system/etc/apns-conf.xml
+    vendor/omni/prebuilt/etc/apns-conf.xml:$(TARGET_COPY_OUT_SYSTEM)/etc/apns-conf.xml
 
 # SIM Toolkit
 PRODUCT_PACKAGES += \