ftrace: Declare function_trace_op in header to quiet sparse warning

Sparse complains that function_trace_op is not static but is not declared
in a header file. It is used only in assembly code. But add it to a header
so that sparse no longer complains:

 kernel/trace/ftrace.c:99:19: warning: symbol 'function_trace_op' was not declared. Should it be static?

Link: https://lore.kernel.org/linux-trace-kernel/20240605202708.289105647@goodmis.org

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

include/linux/ftrace.h

@@ -1131,6 +1131,9 @@ extern void ftrace_graph_init_task(struct task_struct *t);
 extern void ftrace_graph_exit_task(struct task_struct *t);
 extern void ftrace_graph_init_idle_task(struct task_struct *t, int cpu);
 
+/* Used by assembly, but to quiet sparse warnings */
+extern struct ftrace_ops *function_trace_op;
+
 static inline void pause_graph_tracing(void)
 {
 	atomic_inc(&current->tracing_graph_pause);