Make C API compatible with -Wstrict-prototypes

Change-Id: I265605e769a12a319d672c491c9ba2d89c02de7e
Reviewed-on: https://skia-review.googlesource.com/6477
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
5 files changed