ghc-7.4.2-binary: fix on i686 after 6c4364041

I'm not sure what was the compatibility problem before that commit,
but ghc at least builds now on both x86 Linux platforms
(and this commit doesn't cause a rebuild on x86_64-linux).

pkgs/development/compilers/ghc/7.4.2-binary.nix

···
     # find editline/gmp.
     stdenv.lib.optionalString stdenv.isLinux ''
       mkdir -p "$out/lib"
-      ln -sv "${ncurses}/lib/libncurses.so" "$out/lib/libncursesw.so.5"
+      ln -sv "${ncurses}/lib/libncurses.so" "$out/lib/libncurses${stdenv.lib.optionalString stdenv.is64bit "w"}.so.5"
       find . -type f -perm +100 \
           -exec patchelf --interpreter "$(cat $NIX_CC/nix-support/dynamic-linker)" \
           --set-rpath "$out/lib:${gmp}/lib" {} \;