perf x86 kvm-stat: Add missing string.h header

It uses strcmp(), strstr() and was getting the required string.h header
by luck, from evsel.h -> cpumap.h -> debug.h -> string.h, add the
missing header.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-qrz8hhvrhwnmt5ocfwk4br5d@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>