[0.10] add get_backend for StaticArrays

[vchuravy]

Port of #621

[christiangnrd]

Guess we need to find an alternative to MArray for KA.scratchpad or re-add StaticArrays.jl as a full dependency.

KernelAbstractions.jl/src/pocl/backend.jl

Lines 224 to 226 in 4409708

	@device_override @inline function KA.Scratchpad(ctx, ::Type{T}, ::Val{Dims}) where {T, Dims}
	StaticArrays.MArray{KA.__size(Dims), T}(undef)
	end