The recent replacement of <termios.h> with <linux/termios.h> to get
TCGETS2 seems to work only with compilers (or C standards) that allow
the same structure to be defined multiple times. There is a conflict
between <sys/ioctl.h> and <linux/termios.h>.
Another problem is that TCGETS2 is not used on some archs like ppc64.
Switch back to <termios.h> and move TCGETS2 to a list in a separate
file where it can be compiled without <sys/ioctl.h>.
Fixes: 03875f1ea5 ("sys_linux: allow ioctl(TCGETS2) in seccomp filter")
