perf config: Fix build with older toolchain.

Fix build error on Ubuntu 12.04.5 with GCC 4.6.3.

    CC       util/config.o
  util/config.c: In function ‘perf_buildid_config’:
  util/config.c:384:15: error: declaration of ‘dirname’ shadows a global declaration [-Werror=shadow]
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Taeung Song <treeze.taeung@gmail.com>
Cc: Wang Nan <wangnan0@huawei.com>
Fixes: 9cb5987c ("perf config: Rework buildid_dir_command_config to perf_buildid_config")
Link: http://lkml.kernel.org/r/1459807659-9020-1-git-send-email-vlee@freedesktop.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/config.c

@@ -381,11 +381,11 @@ static int perf_buildid_config(const char *var, const char *value)
 {
 	/* same dir for all commands */
 	if (!strcmp(var, "buildid.dir")) {
-		const char *dirname = perf_config_dirname(var, value);
+		const char *dir = perf_config_dirname(var, value);
 
-		if (!dirname)
+		if (!dir)
 			return -1;
-		strncpy(buildid_dir, dirname, MAXPATHLEN-1);
+		strncpy(buildid_dir, dir, MAXPATHLEN-1);
 		buildid_dir[MAXPATHLEN-1] = '\0';
 	}