termios: fix pid_t always, not just for tcgetsid tests-termios.c was failing on more than just cygwin. * doc/posix-headers/termios.texi (termios.h): Mention problem. * lib/termios.in.h (include): Ensure pid_t on all platforms, not just when building tcgetsid. Signed-off-by: Eric Blake <eblake@redhat.com>