powerpc/security: Make symbol 'stf_barrier' static

The sparse tool complains as follows:

arch/powerpc/kernel/security.c:253:6: warning:
 symbol 'stf_barrier' was not declared. Should it be static?

This symbol is not used outside of security.c, so this commit marks it
static.
Signed-off-by: Li Huafei <lihuafei1@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210408033951.28369-1-lihuafei1@huawei.com

arch/powerpc/kernel/security.c

@@ -252,7 +252,7 @@ ssize_t cpu_show_spectre_v2(struct device *dev, struct device_attribute *attr, c
 
 static enum stf_barrier_type stf_enabled_flush_types;
 static bool no_stf_barrier;
-bool stf_barrier;
+static bool stf_barrier;
 
 static int __init handle_no_stf_barrier(char *p)
 {