"ctrl-shrink" svg is not defined

[mourner]

@kkaefer When building CSS in the current master (after #8874), there's a warning:

⚠  "ctrl-shrink" svg is not defined [postcss-inline-svg]

If you look into generated mapbox-gl.css, the corresponding class is not processed, resulting in an invalid CSS rule:

.mapboxgl-ctrl button.mapboxgl-ctrl-shrink .mapboxgl-ctrl-icon{
    background-image:svg-inline(ctrl-shrink)
}