Remove unused mut from long-linker-command-lines test

Author: tmiasko

src/test/run-make-fulldeps/long-linker-command-lines/foo.rs

@@ -90,7 +90,7 @@ fn main() {
         }
 
         let linker_args = read_linker_args(&ok);
-        for mut arg in linker_args.split('S') {
+        for arg in linker_args.split('S') {
             expected_libs.remove(arg);
         }