Add missing OMP ifdef

Author: maliasadi

pennylane_lightning/core/src/simulators/lightning_qubit/utils/LinearAlgebra.hpp

@@ -82,6 +82,7 @@ template <class T,
 inline static void
 omp_innerProd(const std::complex<T> *v1, const std::complex<T> *v2,
               std::complex<T> &result, const size_t data_size) {
+#if defined(_OPENMP)
 #pragma omp declare \
             reduction (sm:std::complex<T>:omp_out=ConstSum(omp_out, omp_in)) \
             initializer(omp_priv=std::complex<T> {0, 0})