Don't generate #[no_mangle] within extern blocks

[kkysen]

Currently, c2rust transpile generates #[no_mangle] on all foreign functions declared within extern blocks. This is not needed, as symbol names in extern blocks are not mangled to begin with. Currently, this is just a #[warn(unused_attributes)] warning, but it will become a hard error in later versions of rustc, so we should stop emitting it.

Here is an example warnings:

warning: `#[no_mangle]` has no effect on a foreign function
 --> src/bin/converted_hello.rs:6:1
  |
6 | #[no_mangle]
  | ^^^^^^^^^^^^ help: remove this attribute
7 | fn printf(_: *const libc::c_char, _: ...) -> libc::c_int;
  | --------------------------------------------------------- foreign function
  |
  = note: `#[warn(unused_attributes)]` on by default
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: symbol names in extern blocks are not mangled