fixed duplicate interface variable in SPIR-V output

BUG=skia:

Change-Id: Iee6b8de83b8ee6ce9de910a12bd86042018d7965
Reviewed-on: https://skia-review.googlesource.com/9167
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
1 file changed