serialport.c: Add missing linux_termios.h #include.

This is safe to include (at the moment) on all non-win32 OSes, since it
doesn't contain anything OS-specific.

serialport.c

@@ -50,6 +50,10 @@
 #endif
 #endif
 
+#ifndef _WIN32
+#include "linux_termios.h"
+#endif
+
 #include "libserialport.h"
 
 struct port_data {