trinity: fix building with uClibc

trinity: fix building with uClibc

"#include <linux/serial.h>" is needed in order to build trinity
successfully with uClibc. Otherwise it would fail displaying an error
message like this one:

-----------------------------------------
ioctls/vt.c:152:8: error: invalid application of 'sizeof' to incomplete
type 'struct serial_rs485'
  IOCTL(TIOCGRS485),

ioctls/vt.c:155:8: error: invalid application of 'sizeof' to incomplete
type 'struct serial_rs485'
  IOCTL(TIOCSRS485),
-----------------------------------------

Backporting an upstream patch to fix this problem.

Upstream commit:

  https://github.com/kernelslacker/trinity/commit/fb4a1adc4540f0702b84aa900f2b8ebed004885d

Fixes:

  http://autobuild.buildroot.net/results/ca1/ca18c7d19ac2870d6b973923c8225509a12f831a/

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>