Add a missing header include, to fix a build error.

Signed-off-by: Simon Dawson <spdawson@gmail.com>

diff -Nurp a/src/SerialPort.cpp b/src/SerialPort.cpp
--- a/src/SerialPort.cpp	2008-11-12 01:27:30.000000000 +0000
+++ b/src/SerialPort.cpp	2013-04-23 15:43:41.537495852 +0100
@@ -27,6 +27,7 @@
 #include <cassert>
 #include <termios.h>
 #include <fcntl.h>
+#include <unistd.h>
 #include <sys/ioctl.h>
 #include <sys/time.h>
 #include <signal.h>