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?
```