perf tools: Update makefile message for installing slang devel package

In case of missing library (libslang), give hint to install library
(libslang2-dev), since libslang-dev is not provided by Ubuntu.

Signed-off-by: Neeraj Badlani <neerajbadlani@gmail.com>
Link: http://lkml.kernel.org/r/1467035997-9100-1-git-send-email-neerajbadlani@gmail.com


[ removed excessive 'or' usage ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>