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: e2ac83d7 ("x86/ftrace: Fix ORC unwinding from ftrace handlers")
Reported-by: Steven Rostedt <rostedt@goodmis.org>
Reported-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lkml.kernel.org/r/20180128022150.dqierscqmt3uwwsr@treble