include: update log2 header from the Linux kernel

Without the patch gcc 8 produces:
warning: ignoring attribute ‘noreturn’ because it conflicts with
attribute ‘const’ [-Wattributes]
 int ____ilog2_NaN(void);

So let's update the include from Linux kernel v4.16.

This removes static checks of ilog2() arguments.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>