graphics/svgbob: Unbreak with Rust 1.37.0

error: trait objects without an explicit dyn are deprecated

--> src/main.rs:163:47
 |

163 | fn build(args: &ArgMatches) -> Result<(), Box<Error>> {

|                                               ^^^^^ help: use `dyn`: `dyn Error`
|

note: lint level defined here

--> src/main.rs:1:9
 |

1 | #![deny(warnings)]

|         ^^^^^^^^
= note: #[deny(bare_trait_objects)] implied by #[deny(warnings)]

error: trait objects without an explicit dyn are deprecated

--> src/main.rs:224:68
 |

224 | fn convert_file(input: PathBuf, output: PathBuf) -> Result<(), Box<Error>> {

|                                                                    ^^^^^ help: use `dyn`: `dyn Error`

error: aborting due to 2 previous errors

Remove #![deny(warnings)] (basically the equivalent to -Werror in
C/C++) to fix the build with Rust 1.37.0 or greater.