fix build on nightly rust

I was getting this:

error[E0106]: missing lifetime specifier
  --> base/time.rs:26:68
   |
26 | fn fixed_len_num<'a>(len: usize) -> impl FnMut(&'a str) -> IResult<&'a str, i32> {
   |                                                                    ^ expected named lifetime parameter
   |
   = help: this function's return type contains a borrowed value with an elided lifetime, but the lifetime cannot be derived from the arguments
help: consider using the `'a` lifetime

with rustc 1.62.0-nightly (a707f4010 2022-04-29).

scottlamb / moonfire-nvr

fix build on nightly rust #222