perf kvm: Move kvm-stat header file from conditional inclusion to common include section

Move kvm-stat header file to the common include section, and make the
definitions in the header file under the conditional inclusion `#ifdef
HAVE_KVM_STAT_SUPPORT`.

This helps to define other 'perf kvm' related function prototypes in
kvm-stat header file, which may not need kvm-stat support.

Signed-off-by: Anju T Sudhakar <anju@linux.vnet.ibm.com>
Reviewed-By: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linuxppc-dev@lists.ozlabs.org
Link: http://lore.kernel.org/lkml/20190718181749.30612-1-anju@linux.vnet.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>