uart_register_input()'s signature changed with the addition of a
callback. gdb_server.c was never updated so now the compilation fails
when enabling the GDB server.
Fix by setting a NULL completion (as seems the code would not use it
anyway).
Change-Id: I4c0df65a31bd906db704f7f4a564e7d6f065aae7
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>
9b3b36bd7a