Set flag exporter provider for vndk_prebuilt libs

vndk_prebuilt exports some flags, so the provider should be set.

Bug: N/A
Test: m
Change-Id: I6c4df4139e66f992b63686adfe0635f08bafa649
diff --git a/cc/vndk_prebuilt.go b/cc/vndk_prebuilt.go
index 7c47ef4..3556d94 100644
--- a/cc/vndk_prebuilt.go
+++ b/cc/vndk_prebuilt.go
@@ -169,6 +169,8 @@
 			TableOfContents: p.tocFile,
 		})
 
+		p.libraryDecorator.flagExporter.setProvider(ctx)
+
 		return in
 	}