perf ftrace: Improve error message about capability to use ftrace

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.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>