lib/vsprintf: Make function pointer_string static

Fix sparse warning:

lib/vsprintf.c:673:6: warning:
 symbol 'pointer_string' was not declared. Should it be static?

Link: http://lkml.kernel.org/r/20190426164630.22104-1-yuehaibing@huawei.com
To: <sergey.senozhatsky@gmail.com>
To: <andriy.shevchenko@linux.intel.com>
To: <geert+renesas@glider.be>
To: <rostedt@goodmis.org>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>

lib/vsprintf.c

@@ -670,8 +670,9 @@ char *string(char *buf, char *end, const char *s,
 	return string_nocheck(buf, end, s, spec);
 }
 
-char *pointer_string(char *buf, char *end, const void *ptr,
-		     struct printf_spec spec)
+static char *pointer_string(char *buf, char *end,
+			    const void *ptr,
+			    struct printf_spec spec)
 {
 	spec.base = 16;
 	spec.flags |= SMALL;