Add a test regular variadic functions errors

Add a new regression test for the error message in regular function
variadic errors during ast validation pass.

gcc/testsuite/ChangeLog:

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

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

gcc/testsuite/rust/compile/non_foreign_variadic_function.rs

@@ -0,0 +1,4 @@
+pub fn toto(a: i32, ...) {}
+// { dg-error "only foreign or .unsafe extern .C.. functions may be C-variadic" "" { target *-*-* } .-1 }
+
+fn main() {}