perf tui: Add workaround for slang < 2.1.4

Older versions of the slang library didn't used the 'const' specifier,
causing problems with modern compilers of this kind:

util/newt.c:252: error: passing argument 1 of ‘SLsmg_printf’ discards
qualifiers from pointer target type

Fix it by using some wrappers that when needed const the affected
parameters back to plain (char *).

Reported-by: Lin Ming <ming.m.lin@intel.com>
Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Lin Ming <ming.m.lin@intel.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Tom Zanussi <tzanussi@gmail.com>
LKML-Reference: <20100517145421.GD29052@ghostprotocols.net>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>