perf demangle-java: Constify variables storing the result of strchr() on const tables

As newer glibcs will propagate the const attribute of the searched table
to its return.

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

tools/perf/util/demangle-java.c

@@ -158,7 +158,7 @@ char *
 java_demangle_sym(const char *str, int flags)
 {
 	char *buf, *ptr;
-	char *p;
+	const char *p;
 	size_t len, l1 = 0;
 
 	if (!str)