blender: add `libwebp` to `buildInputs` (#208201)

Enables the WebP image format support in Blender. It has been [supported](https://developer.blender.org/rB4fd0a69d7ba) since Blender 3.2 but was disabled due to libwebp missing from the build inputs.

    $ nix build nixpkgs#blender
    $ nix log (realpath result)
    ...
    -- Could NOT find WebP (missing: _webp_LIBRARIES WEBP_INCLUDE_DIR)
    CMake Warning at build_files/cmake/platform/platform_unix.cmake:407 (message):
      WebP not found, disabling WITH_IMAGE_WEBP
    Call Stack (most recent call first):
      CMakeLists.txt:1058 (include)

0 files changed, 0 insertions, 0 deletions