So that we get a proper warning in the TUI in cases like:

 $ perf report --stdio -g fractal,0.5,caller --sort pid
 Selected -g but no callchain data. Did you call 'perf record' without -g?
 $

The --stdio case is ok because it uses fprintf, ui__warning is needed to
figure out if --stdio or --tui is being used.

Cc: Arun Sharma <asharma@fb.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sam Liao <phyomh@gmail.com>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-ag9fz2wd17mbbfjsbznq1wms@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>