Include <cmath> in HeavisideImpl kernels

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

@@ -15,6 +15,7 @@
 #include "aidge/utils/Registrar.hpp"
 
 #include <cstddef> // std::size_t
+#include <cmath>
 
 #include "aidge/backend/cpu/operator/HeavisideImpl.hpp"
 #include "aidge/utils/ErrorHandling.hpp"
@@ -37,7 +38,7 @@ void HeavisideImplCpuForwardKernel(std::size_t inputLength,
 
 // Surrogate Gradient
 template <class O, class GO, class GI>
-void HeavisideImplCpuBackwardKernel(std::size_t inputLength, 
+void HeavisideImplCpuBackwardKernel(std::size_t inputLength,
                                     const void* output_,
                                     const void* grad_output_,
                                     void* grad_input_) {