x86/ftrace: Add one more ENDPROC annotation

x86/ftrace: Add one more ENDPROC annotation

When ORC support was added for the ftrace_64.S code, an ENDPROC
for function_hook() was missed. This results in the following warning:

  arch/x86/kernel/ftrace_64.o: warning: objtool: .entry.text+0x0: unreachable instruction

Fixes: e2ac83d74a4d ("x86/ftrace: Fix ORC unwinding from ftrace handlers")
Reported-by: Steven Rostedt <[email protected]>
Reported-by: Borislav Petkov <[email protected]>
Signed-off-by: Josh Poimboeuf <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Acked-by: Ingo Molnar <[email protected]>
Cc: Linus Torvalds <[email protected]>
Link: https://lkml.kernel.org/r/20180128022150.dqierscqmt3uwwsr@treble