perf arm pmu: Add missing linux/string.h header

It uses strstarts(), that is defined in linux/string.h but that was
being including by sheer luck, indirectly, fix it.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Dongjiu Geng <gengdongjiu@huawei.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kim Phillips <kim.phillips@arm.com>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Suzuki Poulouse <suzuki.poulose@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Link: https://lkml.kernel.org/n/tip-vub5lp82wb7vy5wssfad0xu8@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>