Fix wrong webpmux library name

--- a/src/webp.imageio/CMakeLists.txt	2025-11-17 13:37:13.795979999 +0300
+++ b/src/webp.imageio/CMakeLists.txt	2025-11-17 13:37:33.095764587 +0300
@@ -4,7 +4,7 @@
 
 if (WebP_FOUND)
     add_oiio_plugin (webpinput.cpp webpoutput.cpp
-                     LINK_LIBRARIES WebP::webp WebP::webpdemux WebP::libwebpmux
+                     LINK_LIBRARIES WebP::webp WebP::webpdemux WebP::webpmux
                      DEFINITIONS "USE_WEBP=1")
 else ()
     message (STATUS "WebP plugin will not be built")