add missing "static"

This allows us to link both MoltenVK
and the backend which must not be named.

Change-Id: I576aab1aab92a39a4da75d5cd53c6a2c4b5b79fb
Reviewed-on: https://skia-review.googlesource.com/c/161580
Commit-Queue: Mike Klein <mtklein@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
4 files changed