skip runtime registration on iOS

These encoders and decoders are not built and linked
in Google3 iOS builds.  We just need to hack a bit to
get DM to build there... we never run it.

Change-Id: I6cdfbb6df3c4fe30096b2715cad004bccec0bcaa
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/214002
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
1 file changed