Make the return type of rb_char_next the same as CharNext

eval_intern.h

@@ -303,14 +303,14 @@ VALUE rb_ec_backtrace_location_ary(const rb_execution_context_t *ec, long lev, l
 #ifndef CharNext		/* defined as CharNext[AW] on Windows. */
 # ifdef HAVE_MBLEN
 #  define CharNext(p) rb_char_next(p)
-static inline const char *
+static inline char *
 rb_char_next(const char *p)
 {
     if (p) {
         int len = mblen(p, RUBY_MBCHAR_MAXSIZE);
         p += len > 0 ? len : 1;
     }
-    return p;
+    return (char *)p;
 }
 # else
 #  define CharNext(p) ((p) + 1)