perf tools: fix NO_GTK2 Makefile config error

In case the user specified NO_GTK2 on the make cmdline, compilation
would fail with undefined symbol because the Makefile would not set the
correct cpp variable: NO_GTK2 vs. NO_GTK2_SUPPORT.

This patch renames the variable to the correct name.

Signed-off-by: Stephane Eranian <eranian@google.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20120410103513.GA9229@quad


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>