fix "ftrace: store mcount address in rec->ip"

fix "ftrace: store mcount address in rec->ip"

Alexander Beregalov reported this build failure:

$ make CROSS_COMPILE=sparc64-unknown-linux-gnu- image modules && sudo
make modules_install
  CHK     include/linux/version.h
  CHK     include/linux/utsrelease.h
  CALL    scripts/checksyscalls.sh
  CHK     include/linux/compile.h
dnsdomainname: Unknown host
  CC      arch/sparc64/kernel/sparc64_ksyms.o
arch/sparc64/kernel/sparc64_ksyms.c:116: error: '_mcount' undeclared
here (not in a function)
cc1: warnings being treated as errors
arch/sparc64/kernel/sparc64_ksyms.c:116: error: type defaults to 'int'
in declaration of '_mcount'

And bisected it back to:

| commit 395a59d0f8e86bb39cd700c3d185d30c670bb958
| Author: Abhishek Sagar <sagar.abhishek@gmail.com>
| Date:   Sat Jun 21 23:47:27 2008 +0530
|
|     ftrace: store mcount address in rec->ip

the mcount prototype is only available under CONFIG_FTRACE,
extend it to CONFIG_MCOUNT as well.

Reported-and-bisected-by: Alexander Beregalov <a.beregalov@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>