perf ftrace: Improve error message about capability to use ftrace

If we link against libcap, then we can state that CAP_SYS_ADMIN is
needed, if not, fallback to telling the user it needs to be root, as was
before linking against libcap.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexey Budankov <alexey.budankov@linux.intel.com>
Cc: Igor Lubashev <ilubashe@akamai.com>
Cc: James Morris <jmorris@namei.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Suzuki Poulouse <suzuki.poulose@arm.com>
Link: https://lkml.kernel.org/n/tip-hhnbjdo8r67054of9zm2kxtl@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-ftrace.c

@@ -284,7 +284,13 @@ static int __cmd_ftrace(struct perf_ftrace *ftrace, int argc, const char **argv)
 	};
 
 	if (!perf_cap__capable(CAP_SYS_ADMIN)) {
-		pr_err("ftrace only works for root!\n");
+		pr_err("ftrace only works for %s!\n",
+#ifdef HAVE_LIBCAP_SUPPORT
+		"users with the SYS_ADMIN capability"
+#else
+		"root"
+#endif
+		);
 		return -1;
 	}