[TBDGen][Tests] Add apple vendor requirement on package test

TBD files only encode darwin based platforms. This fixes up failing linux bots.

resolves: rdar://118461540

Author: cyndyishida

test/TBD/package_symbol_with_default_arg.swift

@@ -1,3 +1,4 @@
+// REQUIRES: VENDOR=apple 
 // RUN: %empty-directory(%t)
 // RUN: split-file %s %t