tools include: Adopt strstarts() from the kernel

Replacing prefixcmp(), same purpose, inverted result, so standardize on
the kernel variant, to reduce silly differences among tools/ and the
kernel sources, making it easier for people to work in both codebases.

And then doing:

	if (strstarts(option, "no-"))

Looks clearer than doing:

	if (!prefixcmp(option, "no-"))

To figure out if option starts witn "no-".

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: Rusty Russell <rusty@rustcorp.com.au>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-kaei42gi7lpa8subwtv7eug8@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/tools/perf/util/callchain.c b/tools/perf/util/callchain.c
index 22d413a..02130e2 100644
--- a/tools/perf/util/callchain.c
+++ b/tools/perf/util/callchain.c
@@ -304,7 +304,7 @@ int perf_callchain_config(const char *var, const char *value)
 {
 	char *endptr;
 
-	if (prefixcmp(var, "call-graph."))
+	if (!strstarts(var, "call-graph."))
 		return 0;
 	var += sizeof("call-graph.") - 1;