It uses strstr(), needs to include string.h or its not going to build
when we remove string.h from the place it is getting from indirectly, by
luck.

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-72y0i0uiaqght5b83e3ae7p4@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>