scons: Re-enable TRACING_ON flag

The TRACING_ON flag was removed in a previous commit [1], but is still
used by the _check_tracing() function in main.py. This breaks gem5
simulations when debug flags are enabled.

This patch re-enables the TRACING_ON flag.

[1] https://gem5-review.googlesource.com/c/public/gem5/+/48379

Change-Id: I90ed8a46938fa2748b96c1b378329a4ba1ef047e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/50427
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/python/pybind11/core.cc

@@ -289,6 +289,8 @@ pybind_init_core(py::module_ &m_native)
     m_core.attr("compileDate") = py::cast(compileDate);
     m_core.attr("gem5Version") = py::cast(gem5Version);
 
+    m_core.attr("TRACING_ON") = py::cast(TRACING_ON);
+
     m_core.attr("MaxTick") = py::cast(MaxTick);
 
     /*