Fix compilation with GCC 5.4.0 (#117)

This was spotted here: https://travis-ci.org/MRChemSoft/mrchem/jobs/653411396#L559

Author: robertodr

src/XCFunctional.hpp

@@ -31,7 +31,7 @@ struct XCFunctional {
   int depends{0}; // XC_DENSITY, gradient etc
   xcfun_mode mode{XC_MODE_UNSET};
   xcfun_vars vars{XC_VARS_UNSET};
-  std::array<functional_data *, XC_NR_FUNCTIONALS> active_functionals{nullptr};
+  std::array<functional_data *, XC_NR_FUNCTIONALS> active_functionals{{nullptr}};
   std::array<double, XC_NR_PARAMETERS_AND_FUNCTIONALS> settings;
 };