s390/oprofile: fix compile error

Fix these errors when compiling with CONFIG_OPROFILE=y and
CONFIG_PERF_EVENTS=n:
arch/s390/oprofile/init.c: In function ‘oprofile_hwsampler_start’:
arch/s390/oprofile/init.c:93:2: error: implicit declaration of function 'perf_reserve_sampling' [-Werror=implicit-function-declaration]
  retval = perf_reserve_sampling();
  ^
arch/s390/oprofile/init.c:99:3: error: implicit declaration of function 'perf_release_sampling' [-Werror=implicit-function-declaration]
   perf_release_sampling();
   ^

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>