tools lib string: Adopt prefixcmp() from perf and subcmd

Both had copies originating from git.git, move those to
tools/lib/string.c, getting both tools/lib/subcmd/ and tools/perf/ to
use it.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-uidwtticro1qhttzd2rkrkg1@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/subcmd/help.c

@@ -1,6 +1,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <linux/string.h>
 #include <termios.h>
 #include <sys/ioctl.h>
 #include <sys/types.h>