ftrace/samples: Add missing prototype for my_direct_func

ftrace/samples: Add missing prototype for my_direct_func

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

  >> samples/ftrace/ftrace-direct-multi.c:8:6: warning: no previous
  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.

Link: https://lkml.kernel.org/r/[email protected]
Reported-by: kernel test robot <[email protected]>
Fixes: 5fae941b9a6f ("ftrace/samples: Add multi direct interface test module")
Signed-off-by: Jiri Olsa <[email protected]>
Signed-off-by: Steven Rostedt (VMware) <[email protected]>