sched/core: fix have_sched_energy_data build warning

have_sched_energy_data is defined only for CONFIG_SMP, so declare it
only with CONFIG_SMP.

Fixes warning from intel bot:

tree:   https://android.googlesource.com/kernel/msm

 android-4.4
head:   a2129978
commit: a2129978 [5/5] sched/core: Warn
if ENERGY_AWARE is enabled but data is missing
config: i386-randconfig-x002-201743 (attached as .config)
compiler: gcc-6 (Debian 6.2.0-3) 6.2.0 20160901
reproduce:
        git checkout a2129978
        # save the attached .config to linux build tree
        make ARCH=i386

All warnings (new ones prefixed by >>):

>> kernel//sched/core.c:94:13: warning: 'have_sched_energy_data' used
but never defined
    static bool have_sched_energy_data(void);
                ^~~~~~~~~~~~~~~~~~~~~~

vim +/have_sched_energy_data +94 kernel//sched/core.c

    93
  > 94  static bool have_sched_energy_data(void);
    95

Change-Id: I266b63ece6fb31d2b5b11821a8244e147ba6d3a4
Signed-off-by: Joel Fernandes <joelaf@google.com>