Promote spiffxp to build/ approver

I've been a reviewer here for nearly 2 years, and am now part of
the team that builds the google-hosted .dep and .rpm packages

Author: spiffxp

build/OWNERS

@@ -14,6 +14,7 @@ approvers:
   - justaugustus
   - lavalamp
   - mikedanese
+  - spiffxp
 emeritus_approvers:
   - jbeda
   - zmerlynn