perf: Fix missing strndup declaration

<ctype.h> is included first without _GNU_SOURCE, so it ends up
including <string.h> without declaring strndup(). And further
<string.h> declarations, even with _GNU_SOURCE defined, are
of course without effect.

Therefore:

	util/strfilter.c: Dans la fonction «strfilter_node__new» :
	util/strfilter.c:134: attention : déclaration implicite de la fonction « «strndup» »
	util/strfilter.c:134: attention : incompatible implicit declaration of built-in function «strndup»
	make: *** [util/strfilter.o] Erreur 1

Just don't include ctype.h as it doesn't appear to be necessary
anyway.
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Tom Zanussi <tzanussi@gmail.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/strfilter.c

-#include <ctype.h>
 #include "util.h"
 #include "string.h"
 #include "strfilter.h"