x86/bugs: Mark retbleed_strings static

This symbol is not used outside of bugs.c, so mark it static.
Reported-by: NAbaci Robot <abaci@linux.alibaba.com>
Signed-off-by: NJiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: NBorislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220714072939.71162-1-jiapeng.chong@linux.alibaba.com

arch/x86/kernel/cpu/bugs.c

@@ -793,7 +793,7 @@ enum retbleed_mitigation_cmd {
 	RETBLEED_CMD_IBPB,
 };
 
-const char * const retbleed_strings[] = {
+static const char * const retbleed_strings[] = {
 	[RETBLEED_MITIGATION_NONE]	= "Vulnerable",
 	[RETBLEED_MITIGATION_UNRET]	= "Mitigation: untrained return thunk",
 	[RETBLEED_MITIGATION_IBPB]	= "Mitigation: IBPB",