Add back in mat.jl

Author: Jeremy E Kozdon

src/PETSc.jl

@@ -23,7 +23,7 @@ include("init.jl")
 include("viewer.jl")
 include("options.jl")
 include("vec.jl")
-# include("mat.jl")
+include("mat.jl")
 # include("matshell.jl")
 # include("ksp.jl")
 # include("ref.jl")