GPUCommon: place the GPU common constants in the constant address space

MSL requires every variable at program scope, class-scope statics included, to
name an address space. GPUglobalconstexpr() expands to constexpr everywhere
except Metal, so this is a no-op for host, CUDA, HIP, OpenCL and cling.

These were missed by the earlier pass: the header they sit in only started
being compiled once the errors ahead of it were gone.

Author: ktf

GPU/Common/GPUCommonConstants.h

@@ -19,8 +19,8 @@
 
 namespace o2::gpu::gpu_common_constants
 {
-static constexpr const float kCLight = 0.000299792458f; // TODO: Duplicate of MathConstants, fix this now that we use only OpenCL CPP
-static constexpr const float kZeroFieldCut = 0.013f;
+static GPUglobalconstexpr() const float kCLight = 0.000299792458f; // TODO: Duplicate of MathConstants, fix this now that we use only OpenCL CPP
+static GPUglobalconstexpr() const float kZeroFieldCut = 0.013f;
 }
 
 #endif