Don't check library variant identicalness for VNDK extensions

These libraries don't have a core variant against which to check
for identicalness.

Bug: 148526685
Test: built a previously failing target with this change (succeeded)
Change-Id: Ide8ec58df1868175f52c005bf73bb81fc196a571
diff --git a/cc/library.go b/cc/library.go
index 99a3e16..6c8f5bf 100644
--- a/cc/library.go
+++ b/cc/library.go
@@ -1104,7 +1104,7 @@
 			if ctx.isVndkSp() {
 				library.baseInstaller.subDir = "vndk-sp"
 			} else if ctx.isVndk() {
-				if !ctx.mustUseVendorVariant() {
+				if !ctx.mustUseVendorVariant() && !ctx.isVndkExt() {
 					library.checkSameCoreVariant = true
 					if ctx.DeviceConfig().VndkUseCoreVariant() {
 						library.useCoreVariant = true