Stop @cxxdereference from messing up linenum information

[omus]

Makes a minor modification to @cxxdeference to stop that macro from add in line number information which blocks convenient macros likes @which and @edit from working properly:

julia> using CxxWrap

julia> foo() = 1
foo (generic function with 1 method)

julia> @which foo()
foo()
     @ Main REPL[2]:1

julia> @cxxdereference foo() = 1
foo (generic function with 1 method)

julia> @which foo()
foo()
     @ Main ~/.julia/packages/CxxWrap/VnssN/src/CxxWrap.jl:846

julia> @macroexpand @cxxdereference foo() = 1
:(function foo(; )
      #= /Users/cvogt/.julia/packages/CxxWrap/VnssN/src/CxxWrap.jl:846 =#
      #= REPL[8]:1 =#
      1
  end)

With this PR:

julia> using CxxWrap

julia> @cxxdereference foo() = 1
foo (generic function with 1 method)

julia> @which foo()
foo()
     @ Main REPL[2]:1

[barche]

Thanks!