Parse unnamed C varargs within function pointer types

[dtolnay]

error: expected one of: `for`, parentheses, `fn`, `unsafe`, `extern`, identifier, `::`, `<`, `dyn`, square brackets, `*`, `&`, `!`, `impl`, `_`, lifetime
  --> library/std/src/sys/pal/uefi/helpers.rs:24:103
   |
24 | type BootInstallMultipleProtocolInterfaces = unsafe extern "efiapi" fn(_: *mut r_efi::efi::Handle, _: ...) -> r_efi::efi::Status;
   |                                                                                                       ^

[dtolnay]

If the variadic arg is named, it's already correctly parsed. unsafe extern fn(_: T, rest: ...)