audit: mark audit_log_vformat() with __printf() attribute

audit: mark audit_log_vformat() with __printf() attribute

audit_log_vformat() is using printf() type of format, and GCC compiler
(Debian 14.2.0-17) is not happy about this:

kernel/audit.c:1978:9: error: function ‘audit_log_vformat’
  might be a candidate for ‘gnu_printf’ format attribute
kernel/audit.c:1987:17: error: function ‘audit_log_vformat’
  might be a candidate for ‘gnu_printf’ format attribute

Fix the compilation errors (`make W=1` when CONFIG_WERROR=y, which is
default) by adding __printf() attribute.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
[PM: commit description line wrap fixes]
Signed-off-by: Paul Moore <paul@paul-moore.com>