fix libvdwxc test with cuda

If libvdwxc is not available, even in the GPU version of the code, libvdwxc_init() is called for this test.

Test environment

• CPU: Intel® Xeon® Platinum 8360Y
• Libraries: intel/21.2.0 gsl/2.4 libxc/4.3.4 cgal/5.2 cuda/11.2 impi/2021.2 hdf5-serial/1.8.22 metis/5.1 boost/1.74 mkl/2021.2 netcdf-serial/4.4.1 parmetis/4.0 gcc/10
• Configuration options : maxdim3 openmp mpi cuda sse2 avx
• Optional libraries : cgal metis mpi2 netcdf parmetis scalapack
• Architecture : x86_64
• C compiler : mpiicc (icc)
• C compiler flags : -O3 -xCORE-AVX512 -qopt-zmm-usage=high -fma -ip -g -traceback
• C++ compiler : mpiicpc (icpc)
• C++ compiler flags : -O3 -xCORE-AVX512 -qopt-zmm-usage=high -fma -ip -g -traceback -std=c++14
• Fortran compiler : mpiifort (ifort) (Intel(R) Fortran Intel(R) 64 Compiler Classic for applications running on Intel(R) 64, Version 2021.2.0 Build 20210228_000000)
• Fortran compiler flags : -O3 -xCORE-AVX512 -qopt-zmm-usage=high -fma -ip -g -traceback -fopenmp

************************** GPU acceleration **************************
Rank 00000 uses device number 00000
Rank 00001 uses device number 00000

Selected device:
      Framework              : CUDA
      Device type            : GPU
      Device vendor          : NVIDIA Corporation
      Device name            : A100-SXM4-40GB
      Cuda capabilities      : 8.0
      Driver version         : 11020
      Device memory          : 40536.188 MiB
      Local/shared memory    : 48.000 KiB
      Max. group/block size  : 1024

**********************************************************************

error

**************************** FATAL ERROR *****************************
*** Fatal Error (description follows)
*--------------------------------------------------------------------
* From node =    0
*--------------------------------------------------------------------
* Octopus not compiled with libvdwxc
**********************************************************************