Disable all warnings in gdb.rust/traits.rs

Disable all warnings in gdb.rust/traits.rs

With rustc 1.37, I started seeing compiler warnings from the traits.rs
test case:

    warning: trait objects without an explicit `dyn` are deprecated

It seems to me that we generally do not want warnings in these test
cases.  At some point, we'll probably have to patch traits.rs to use
the "dyn" keyword; by that time I expect that all the Rust compilers
in common use will support it.  In the meantime it seemed simplest to
simply disable all warnings in this file.

gdb/testsuite/ChangeLog
2019-09-30  Tom Tromey  <tromey@adacore.com>

	* gdb.rust/traits.rs: Disable all warnings.