Fix GPU breakage in last PR (#520)

Who broke the build? Sam did!

arbor/backends/gpu/multi_event_stream.hpp

@@ -2,6 +2,7 @@
 
 // Indexed collection of pop-only event queues --- multicore back-end implementation.
 
+#include <arbor/arbexcept.hpp>
 #include <arbor/common_types.hpp>
 #include <arbor/fvm_types.hpp>
 #include <arbor/generic_event.hpp>

arbor/backends/gpu/shared_state.cpp

 #include <cstddef>
 #include <vector>
 
+#include <arbor/constants.hpp>
 #include <arbor/fvm_types.hpp>
 #include <arbor/ion.hpp>
 
@@ -8,7 +9,6 @@
 #include "backends/gpu/gpu_store_types.hpp"
 #include "backends/gpu/shared_state.hpp"
 #include "backends/multi_event_stream_state.hpp"
-#include "constants.hpp"
 #include "memory/wrappers.hpp"
 #include "util/rangeutil.hpp"
 

test/unit/test_mc_cell_group_gpu.cpp

@@ -2,7 +2,6 @@
 
 #include <arbor/common_types.hpp>
 
-#include "backends.hpp"
 #include "epoch.hpp"
 #include "fvm_lowered_cell.hpp"
 #include "mc_cell_group.hpp"