Fix dds feature enabling bevy_gltf

[rparrett]

Objective

Fixes #17022

Solution

Only enable bevy_gltf/dds if bevy_gltf is already enabled.

Testing

Tested with empty project

[dependencies]
bevy = { version = "0.16.0-dev", path = "../bevy", default-features = false, features = [
    "dds",
] }

Before

 cargo tree --depth 1 -i bevy_gltf
bevy_gltf v0.16.0-dev (/Users/robparrett/src/bevy/crates/bevy_gltf)
└── bevy_internal v0.16.0-dev (/Users/robparrett/src/bevy/crates/bevy_internal)

After

 cargo tree --depth 1 -i bevy_gltf
warning: nothing to print.

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