Add a new regression test for named variadics

This test ensure that extern C named variadics are parsed correctly.

gcc/testsuite/ChangeLog:

	* rust/compile/extern_c_named_variadic.rs: New test.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>

gcc/testsuite/rust/compile/extern_c_named_variadic.rs

@@ -0,0 +1,5 @@
+extern "C" {
+    fn variadic(x: isize, args: ...);
+}
+
+fn main() {}