vimPlugins.clang_complete: fix path to libclang.so

With the current plugin configuration I get the following error when
opening a C/C++ file in ViM:

```
"main.cpp" 246L, 7522C
Loading libclang failed, completion won't be available. Are you sure '/nix/store/jdz3pbl853dw6k0qg54ifd5fplkq7xl8-cla
ng-3.9.1/lib/libclang.so' contains libclang?
```

pkgs/misc/vim-plugins/overrides.nix

@@ -94,7 +94,7 @@ with generated;
     # $ eval echo $(nix-instantiate --eval --expr 'with (import <nixpkgs>) {}; clang.default_cxx_stdlib_compile')
     preFixup = ''
       substituteInPlace "$out"/share/vim-plugins/clang_complete/plugin/clang_complete.vim \
-        --replace "let g:clang_library_path = '' + "''" + ''" "let g:clang_library_path='${llvmPackages.clang.cc}/lib/libclang.so'"
+        --replace "let g:clang_library_path = '' + "''" + ''" "let g:clang_library_path='${llvmPackages.clang.cc.lib}/lib/libclang.so'"
     '';
   });