Add libwebpdemux to .pc file for static builds

This is required to successfully link to SDL2_image if it was build statically while not using the vendored version of libwebp.

SDL3 port of 1fa2a92

CMakeLists.txt

@@ -923,7 +923,7 @@ if(SDLIMAGE_WEBP)
             message(STATUS "${PROJECT_NAME}: Using system libwebp")
             set(SDLIMAGE_WEBP_ENABLED TRUE)
             if(NOT SDLIMAGE_WEBP_SHARED)
-              list(APPEND PC_REQUIRES libwebp)
+              list(APPEND PC_REQUIRES libwebp libwebpdemux)
               list(APPEND INSTALL_EXTRA_CMAKE_MODULES cmake/Findwebp.cmake)
             endif()
         else()