perf top: Use a macro instead of a constant variable

To overcome a silly gcc warning:

 cc1: warnings being treated as errors
 builtin-top.c: In function ‘lookup_sym_source’:
 builtin-top.c:291: warning: not protecting local variables:
 variable length buffer make: *** [builtin-top.o] Error 1
 make: *** Waiting for unfinished jobs....

That is emitted for this:

	const size_t pattern_len = BITS_PER_LONG / 4 + 2;
	char pattern[pattern_len + 1];

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
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>
LKML-Reference: <1266866062-6287-1-git-send-email-acme@infradead.org>
[ -v2: macroify the naming style ]
Signed-off-by: Ingo Molnar <mingo@elte.hu>