Fix compat

[chriselrod]

Latest release:

julia> @time using LabelledArrays
[ Info: Precompiling LabelledArrays [2ee39098-c373-598a-b85f-a56591580800]
ERROR: LoadError: UndefVarError: undefmatrix not defined
Stacktrace:
 [1] getproperty(x::Module, f::Symbol)
   @ Base ./Base.jl:31
 [2] top-level scope
   @ ~/.julia/packages/LabelledArrays/q5eeO/src/LabelledArrays.jl:69
 [3] include
   @ ./Base.jl:419 [inlined]
 [4] include_package_for_output(pkg::Base.PkgId, input::String, depot_path::Vector{String}, dl_load_path::Vector{String}, load_path::Vector{String}, concrete_deps::Vector{Pair{Base.PkgId, UInt64}}, source::Nothing)
   @ Base ./loading.jl:1554
 [5] top-level scope
   @ stdin:1
in expression starting at /home/chriselrod/.julia/packages/LabelledArrays/q5eeO/src/LabelledArrays.jl:1
in expression starting at stdin:1
ERROR: Failed to precompile LabelledArrays [2ee39098-c373-598a-b85f-a56591580800] to /home/chriselrod/.julia/compiled/v1.8/LabelledArrays/jl_P2I6Yo.

PR:
hopefully it actually loads, I haven't tested.