cygwin: Add missing TIOCM_DSR from termios.h

https://github.com/msys2/msys2-runtime/blob/msys2-3.6.7/winsup/cygwin/include/sys/termios.h#L37

Author: 500-internal-server-error

src/unix/cygwin/mod.rs

@@ -1038,6 +1038,7 @@ pub const TIOCM_RTS: c_int = 0x004;
 pub const TIOCM_CTS: c_int = 0x020;
 pub const TIOCM_CAR: c_int = 0x040;
 pub const TIOCM_RNG: c_int = 0x080;
+pub const TIOCM_DSR: c_int = 0x100;
 pub const TIOCM_CD: c_int = TIOCM_CAR;
 pub const TIOCM_RI: c_int = TIOCM_RNG;
 pub const TCOOFF: c_int = 0;