C: add missing name

Even though I contributed to golang/go[^1], the rule[^2] did not reflect my name in CONTRIBUTORS, so I added it.

[^1]: https://go-review.googlesource.com/c/go/+/164199
[^2]: https://github.com/golang/build/blob/e21a90b/cmd/updatecontrib/updatecontrib.go#L118

Change-Id: I5d6b5684d61ea5da679519a9e703d977470de175
Reviewed-on: https://go-review.googlesource.com/c/go/+/214617Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

CONTRIBUTORS

@@ -1275,6 +1275,7 @@ Koki Tomoshige <tomocy.dev@gmail.com>
 Komu Wairagu <komuw05@gmail.com>
 Konstantin <konstantin8105@gmail.com>
 Konstantin Shaposhnikov <k.shaposhnikov@gmail.com>
+Koya IWAMURA <kiwamura0314@gmail.com>
 Kris Kwiatkowski <kris@cloudflare.com>
 Kris Nova <kris@nivenly.com>
 Kris Rousey <krousey@google.com>