CI fix

`GPUArraysCore` doesnt export `backend`

Author: N5N3

src/StructArrays.jl

@@ -32,7 +32,7 @@ Adapt.adapt_structure(to, s::StructArray) = replace_storage(x->Adapt.adapt(to, x
 # for GPU broadcast
 import GPUArraysCore
 function GPUArraysCore.backend(::Type{T}) where {T<:StructArray}
-    backs = map(backend, fieldtypes(array_types(T)))
+    backs = map(GPUArraysCore.backend, fieldtypes(array_types(T)))
     all(Base.Fix2(===, backs[1]), tail(backs)) || error("backend mismatch!")
     return backs[1]
 end