tracing: Make system_callback() function static

The system_callback() function in trace_events.c is only used within that
file. The "static" annotation was missed.

Fixes: 5790b1fb ("eventfs: Remove eventfs_file and just use eventfs_inode")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202310051743.y9EobbUr-lkp@intel.com/


Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>