add backwards compat

Author: ChrisRackauckas

src/LinearSolve.jl

@@ -124,6 +124,9 @@ end
         @require KrylovKit="0b1a1467-8014-51b9-945f-bf0ae24f4b77" begin
             include("../ext/LinearSolveKrylovKitExt.jl")
         end
+        @require MKL_jll="856f044c-d86e-5d09-b602-aeab76dc8ba7" begin
+            include("../ext/LinearSolveMKLExt.jl")
+        end
     end
 end