tracing: Make system_callback() function static

tracing: Make system_callback() function static

commit 5ddd8baa4857709b4e5d84b376d735152851955b upstream.

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

Fixes: 5790b1fb3d672 ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>