perf symbols: Make variable receiving result strrchr() const

Fixing:

  util/symbol.c: In function ‘symbol__config_symfs’:
  util/symbol.c:2499:20: error: assignment discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
   2499 |         layout_str = strrchr(dir, ',');
        |

With recent gcc/glibc.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

tools/perf/util/symbol.c

@@ -2493,7 +2493,7 @@ int symbol__config_symfs(const struct option *opt __maybe_unused,
 			 const char *dir, int unset __maybe_unused)
 {
 	char *bf = NULL;
-	char *layout_str;
+	const char *layout_str;
 	char *dir_copy;
 	int ret;