perf tools: Fix "==" into "=" in ui_browser__warning assignment

Convert "==" into "=" in ui_browser__warning assignment.

Bug description: https://bugzilla.kernel.org/show_bug.cgi?id=76751



Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Dianfang Zhang <zhangdianfang@huawei.com>
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jean Delvare <jdelvare@suse.de>
Link: http://lkml.kernel.org/r/20140530154709.GC1202@kernel.org


[ changed the changelog a bit ]
Signed-off-by: Jiri Olsa <jolsa@kernel.org>