add isdiag import into default subpackage

Author: ChrisRackauckas

lib/OrdinaryDiffEqDefault/src/OrdinaryDiffEqDefault.jl

@@ -5,7 +5,7 @@ using OrdinaryDiffEq: Vern7, Vern8, Vern9, Vern6, Tsit5, Rosenbrock23, Rodas5P,
     CompositeAlgorithm, OrdinaryDiffEqAlgorithm, OrdinaryDiffEqMutableCache, AutoAlgSwitch
 import OrdinaryDiffEq: is_mass_matrix_alg, default_autoswitch
 import LinearSolve
-using LinearAlgebra: I
+using LinearAlgebra: I, isdiag
 using EnumX
 
 include("default_alg.jl")