GN: Define SK_HAS_... in all Skia-private code, not just libskia.

SkForceLinking wasn't force-linking the WEBP encoder, or any encoder.
(The others must be explicitly used.)

This should help the GN Debug Perf bots progress past

    Running Encode_color_wheel.jpg_WEBP nonrendering
    ../../../bench/EncoderBench.cpp:58: fatal error: "assert(data)"

On my laptop, a debug nanobench run now finishes.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2196243002

Review-Url: https://codereview.chromium.org/2196243002
1 file changed