[eigen-solver] fix warnings about non-void function without return statement

dune/xt/la/eigen-solver/internal/lapacke.hh

@@ -215,6 +215,7 @@ struct lapack_helper
                     "https://software.intel.com/en-us/mkl-developer-reference-c-geev "
                     "and add a corresponding free function like "
                     "compute_eigenvalues_of_a_real_matrix_using_lapack(...)!");
+      return std::vector<std::complex<double>>();
     }
 
     template <class V, class E>

dune/xt/la/matrix-inverter.hh

@@ -36,12 +36,14 @@ class MatrixInverterOptions
   {
     static_assert(AlwaysFalse<MatrixType>::value,
                   "Please implement for given MatrixType and add the respective include below!");
+    return std::vector<std::string>();
   }
 
   static Common::Configuration options(const std::string /*type*/ = "")
   {
     static_assert(AlwaysFalse<MatrixType>::value,
                   "Please implement for given MatrixType and add the respective include below!");
+    return Common::Configuration();
   }
 }; // class MatrixInverterOptions