Add Nilstrieb to mailmap

I saw that I'm on https://thanks.rust-lang.org/ twice (once as
`Nilstrieb` and once as `nils`).

This is because I've contributed as both `Nilstrieb` (locally) and `nils` (on github).
So I've mailmapped them both to `Nilstrieb` (the less ambiguous of the
two).

.mailmap

@@ -415,6 +415,7 @@ Nick Platt <[email protected]>
 Niclas Schwarzlose <[email protected]>
 Nicolas Abram <[email protected]>
 Nicole Mazzuca <[email protected]>
+Nilstrieb <[email protected]> nils <[email protected]>
 Nif Ward <[email protected]>
 Nika Layzell <[email protected]> <[email protected]>
 NODA Kai <[email protected]>