fgraph: Add declaration of "struct fgraph_ret_regs"

In final testing of:

  https://patchwork.kernel.org/project/linux-trace-kernel/patch/1fc502712c981e0e6742185ba242992170ac9da8.1680954589.git.pengdonglin@sangfor.com.cn/
  "function_graph: Support recording and printing the return value of function"

The test failed due to a new warning found in the build:

kernel/trace/fgraph.c:243:56: warning: ‘struct fgraph_ret_regs’ declared inside parameter list will not be visible outside of this definition or declaration

Instead of asking to send another patch series, just add it and then apply
the updates.

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