bootconfig: init: make xbc_namebuf static

This eliminates the following sparse warning:

init/main.c:306:6: warning: symbol 'xbc_namebuf' was not declared.
Should it be static?

Link: https://lkml.kernel.org/r/20200915070324.2239473-1-yanaijie@huawei.comReported-by: Hulk Robot <hulkci@huawei.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

init/main.c

@@ -304,7 +304,7 @@ static void * __init get_boot_config_from_initrd(u32 *_size, u32 *_csum)
 
 #ifdef CONFIG_BOOT_CONFIG
 
-char xbc_namebuf[XBC_KEYLEN_MAX] __initdata;
+static char xbc_namebuf[XBC_KEYLEN_MAX] __initdata;
 
 #define rest(dst, end) ((end) > (dst) ? (end) - (dst) : 0)