perf tools: Substract -Wformat-nonliteral from Wformat=2 in extra flags

The soon coming perf trace needs to use printf with dynamically
built formats.

But we are using -Wformat=2 which is a shortcut for the
following set: -Wformat -Wformat-security -Wformat-y2k
-Wformat-nonliteral

-Wformat-nonliteral warns when it can't check formats because
they are not builtin constant strings, but we want to feature
dynamic formats. What we want instead is Wformat=2 minus
-Wformat-nonliteral, which is what this patch does.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
LKML-Reference: <1250437927-25490-1-git-send-email-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>