vsnprintf: Silence false positive GCC warning for va_format()

vsnprintf: Silence false positive GCC warning for va_format()

va_format() is using vsnprintf(), and GCC compiler (Debian 14.2.0-17)
is not happy about this:

lib/vsprintf.c:1704:9: error: function ‘va_format’ might be a candidate for ‘gnu_print ’ format attribute [-Werror=suggest-attribute=format]

Fix the compilation errors (`make W=1` when CONFIG_WERROR=y, which is default)
by silencing the false positive GCC warning.

Suggested-by: Rasmus Villemoes <ravi@prevas.dk>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Kees Cook <kees@kernel.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20250321144822.324050-7-andriy.shevchenko@linux.intel.com
Signed-off-by: Petr Mladek <pmladek@suse.com>