Only run dsymutil on macos.

I have this approach of bundling dsymutil. If there was
a library that handled bundling DWARF info then this wouldn't be needed.

debugvault/src/lib.rs

@@ -309,6 +309,7 @@ fn sort_by_shortest_match(input: &[&ArcStr], prefix: &str) -> Vec<String> {
  matches
 }
 
+#[cfg(target_os = "macos")]
 pub fn macho_dwarf(obj: &object::File, path: &Path) -> Result<Dwarf, dwarf::Error> {
  let mut dwarf = Dwarf::parse(obj)?;