Fix complex_helper.hpp for CUDA (in terms of not implemeted (yet))

pystencils/include/complex_helper.hpp

@@ -7,6 +7,10 @@
 
 #pragma once
 
+extern "C++" {
+#ifdef __CUDA_ARCH__
+// TODO
+#else
 #include <complex>
 
 template <class U, class V>
@@ -64,3 +68,5 @@ auto operator/(const V &scalar, const std::complex<U> &complexNumber)
   return std::complex<U>{scalar, 0} / complexNumber;
 }
 
+#endif
+}