ftrace/samples: Add missing prototypes direct functions

ftrace/samples: Add missing prototypes direct functions

There's another compilation fail (first here [1]) reported by kernel
test robot for W=1 clang build:

  >> samples/ftrace/ftrace-direct-multi-modify.c:7:6: warning: no previous
  prototype for function 'my_direct_func1' [-Wmissing-prototypes]
     void my_direct_func1(unsigned long ip)

Direct functions in ftrace direct sample modules need to have prototypes
defined. They are already global in order to be visible for the inline
assembly, so there's no problem.

The kernel test robot reported just error for ftrace-direct-multi-modify,
but I got same errors also for the rest of the modules touched by this patch.

[1] 65b16805e5f5 ftrace/samples: Add missing prototype for my_direct_func

Link: https://lkml.kernel.org/r/20211219135317.212430-1-jolsa@kernel.org
Reported-by: kernel test robot <lkp@intel.com>
Fixes: 5436a993f68f ("ftrace/samples: Add module to test multi direct modify interface")
Fixes: 9dac1176cf01 ("ftrace/samples: Add a sample module that implements modify_ftrace_direct()")
Fixes: a1d0e2c7d3a5 ("ftrace: Add another example of register_ftrace_direct() use case")
Fixes: a46b751a1b3c ("ftrace: Add sample module that uses register_ftrace_direct()")
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>