Fix reduce test for custom types

Author: shwina

c/parallel/test/test_reduce.cpp

@@ -77,7 +77,7 @@ TEST_CASE("Reduce works with custom types", "[reduce]")
   operation_t op = make_operation(
     "op",
     "struct pair { short a; size_t b; };\n"
-    "extern \"C\" __device__ op(pair* lhs, pair* rhs, pair* out) {\n"
+    "extern \"C\" __device__ void op(pair* lhs, pair* rhs, pair* out) {\n"
     "  *out = pair{ lhs->a + rhs->a, lhs->b + rhs->b };\n"
     "}");
   const std::vector<short> a  = generate<short>(num_items);