Fixed Windows build

include/aidge/backend/cpu/operator/MatMulImpl_kernels.hpp

@@ -29,7 +29,7 @@ void MatMulImpl_cpu_forward_kernel(const std::size_t n, const std::size_t k, con
 #ifdef _OPENMP
     #pragma omp parallel for if (n > 32)
 #endif
-    for (std::size_t i = 0; i < n; ++i) {
+    for (int i = 0; i < static_cast<int>(n); ++i) {
         for (std::size_t l = 0; l < k; ++l) {
             for (std::size_t j = 0; j < m; ++j) {
                 output[i*m + j] += static_cast<O>(input1[i*k + l] * input2[l*m + j]);