extras: fix WEBP_SWAP_16BIT_CSP check
extras: fix WEBP_SWAP_16BIT_CSP check

this is defined to 0 by dsp.h if it wasn't defined previously, since:
47178dbd extras: add WebPUnmultiplyARGB() convenience function

Change-Id: If4dd48360a95b2786410670cff5ac655227fb6dd

PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp