perf tools: Remove direct slang.h include

We wrap it in libslang.h because we need to deal with older slang release
where HAVE_LONG_LONG is referenced as:

So we need to define it.

Noticed when rebuilding the perf tools on a RHEL5 machine.

Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Stephane Eranian <eranian@google.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>