perf hists browser: Add missing stdarg.h include

perf hists browser: Add missing stdarg.h include

    CC util/ui/browsers/annotate.o
In file included from util/ui/browsers/annotate.c:2:0:
util/ui/browsers/../helpline.h:9:42: error: expected declaration
specifiers or ‘...’ before ‘va_list’
    CC util/ui/browsers/hists.o
make: *** [util/ui/browsers/annotate.o] Error 1
make: *** Waiting for unfinished jobs....

Signed-off-by: Mike Galbraith <efault@gmx.de>
Link: http://lkml.kernel.org/n/tip-9vefl2807smi7t4luhs00tg6@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>