core_timing: Rename CoreTiming namespace to Core::Timing

Places all of the timing-related functionality under the existing Core
namespace to keep things consistent, rather than having the timing
utilities sitting in its own completely separate namespace.

src/core/hle/service/nvdrv/devices/nvhost_ctrl_gpu.cpp

@@ -184,7 +184,7 @@ u32 nvhost_ctrl_gpu::GetGpuTime(const std::vector<u8>& input, std::vector<u8>& o
 
     IoctlGetGpuTime params{};
     std::memcpy(&params, input.data(), input.size());
-    params.gpu_time = CoreTiming::cyclesToNs(CoreTiming::GetTicks());
+    params.gpu_time = Core::Timing::cyclesToNs(Core::Timing::GetTicks());
     std::memcpy(output.data(), &params, output.size());
     return 0;
 }