__attribute__(format(printf)) can also be used for functions that take a
va_list argument.
As per the GCC docs:
For functions where the arguments are not available to be checked
(such as vprintf), specify the third parameter as zero.
Link: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>