x86/bugs: Mark retbleed_strings static

This symbol is not used outside of bugs.c, so mark it static.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220714072939.71162-1-jiapeng.chong@linux.alibaba.com