server: gdb_server: Add colon for target extended-remote

server: gdb_server: Add colon for target extended-remote

Both GDB commands "target remote" and "target extended-remote" require
to have ":" right before port number.

e.g.
    (gdb) target extended-remote :3333

Add ":" to the warning message so that users can copy & past it.

Change-Id: Id6d8ec1e4dfd3c12cb7f3b314064f2c35fa7ab55
Signed-off-by: Yasushi SHOJI <yashi@spacecubics.com>
Reviewed-on: http://openocd.zylin.com/6237
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Tested-by: jenkins
Reviewed-by: Marc Schink <dev@zapb.de>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>