ftrace/samples: Add missing prototype for my_direct_func

There's compilation fail reported kernel test robot for W=1 build:

prototype for function 'my_direct_func' [-Wmissing-prototypes]
void my_direct_func(unsigned long ip)

The inlined assembly is used outside function, so we can't make my_direct_func static and pass it as asm input argument.

However my_tramp is already extern so I think there's no problem keeping my_direct_func extern as well and just add its prototype.

---
 samples/ftrace/ftrace-direct-multi.c | 2 ++
 1 file changed, 2 insertions(+)

index 2a5b1fb7ac14..b6d7806b400e 100644
@@ -5,6 +5,8 @@
 #include <linux/ftrace.h>
 #include <linux/sched/stat.h>
 
+extern void my_direct_func(unsigned long ip);
+
 void my_direct_func(unsigned long ip)
 {
 	trace_printk("ip %lx\n", ip);

2.32.0