x11/swayimg: unbreak build after 7113f719ed96

ld: error: undefined symbol: WebPGetFeaturesInternal

referenced by webp.c

swayimg.p/src_formats_webp.c.o:(decode_webp)

PR: 272437
Reported by: pkg-fallout (quarterly)
(cherry picked from commit 086088b137c0651258ae878f9c63e3acce3a14cd)