Skip unique apex_available enforcement on libz (a stub library)

libz is a stub library, but needs to be available to runtime apex
because it gets statically linked into bionic linker

Bug: 281077552
Bug: 277651159
Test: m nothing
Change-Id: I04f6f13768d8f9c160ce84202e2003b195176355
diff --git a/cc/cc.go b/cc/cc.go
index 9f32c44..7f3490d 100644
--- a/cc/cc.go
+++ b/cc/cc.go
@@ -1941,6 +1941,7 @@
 		"libdl_android": true,
 		"libm":          true,
 		"libdl":         true,
+		"libz":          true,
 		// art apex
 		"libandroidio":    true,
 		"libdexfile":      true,