Add space to fix incorrect macro call.

Previously, `Pkg.add("IJulia")` or `julia deps/build.jl` failed with an error message, the meat of which was
"wrong number of arguments while loading /home/christopher/.julia/v0.3/IJulia/deps/build.jl, in expression starting on line 79".

This was ultimately caused by the fact that the macro name @windows? was immediately followed by an open parenthesis. The parentheses were intended to mark the *first* argument of @windows?---this argument itself is a nontrivial expression---but instead, because there was no space, Julia expected all the arguments to be inside the parentheses. Adding a space fixes this problem (see http://docs.julialang.org/en/release-0.2/manual/metaprogramming/#macros).