Use MakeVar from soong instead of hardcoding

check-vndk-list target reads the vndk.libraries.txt file from
SOONG_VNDK_LIBRARIES_FILE makevar.

Bug: 142963962
Test: m check-vndk-list
Change-Id: I10f6ad80757ed267b66c9a709b75621e063f3744
diff --git a/target/product/gsi/Android.mk b/target/product/gsi/Android.mk
index f9402cb..536fe0c 100644
--- a/target/product/gsi/Android.mk
+++ b/target/product/gsi/Android.mk
@@ -2,7 +2,7 @@
 
 #####################################################################
 # list of vndk libraries from the source code.
-INTERNAL_VNDK_LIB_LIST := $(SOONG_OUT_DIR)/vndk/vndk.libraries.txt
+INTERNAL_VNDK_LIB_LIST := $(SOONG_VNDK_LIBRARIES_FILE)
 
 #####################################################################
 # This is the up-to-date list of vndk libs.