fix OpenCL.UserEvent constructor (retain by default)
fix OpenCL.UserEvent constructor (retain by default)

JuliaPackageMirrors/OpenCL.jl,yuyichao/OpenCL.jl,tkelman/OpenCL.jl,tkelman/OpenCL.jl,yuyichao/OpenCL.jl,JuliaPackageMirrors/OpenCL.jl