E0138 needs to be updated to new format

[sophiajt]

From: src/test/compile-fail/E0138.rs

E0138 needs a span_label, updating it from:

error[E0138]: multiple 'start' functions
  --> src/test/compile-fail/E0138.rs:17:1
   |
17 | fn f(argc: isize, argv: *const *const u8) -> isize {} //~ ERROR E0138
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

To:

error[E0138]: multiple 'start' functions
  --> src/test/compile-fail/E0138.rs:17:1
   |
17 | fn f(argc: isize, argv: *const *const u8) -> isize {} //~ ERROR E0138
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ multiple `start` functions

Bonus: would be better to just underline the name of the function and show the other start function:

error[E0138]: multiple 'start' functions
  --> src/test/compile-fail/E0138.rs:17:1
   |
14 | fn foo(argc: isize, argv: *const *const u8) -> isize {}
        --- previous `start` function here 
...
17 | fn f(argc: isize, argv: *const *const u8) -> isize {} //~ ERROR E0138
   |    ^ multiple `start` functions