ktest.pl: Give back console on Ctrt^C on monitor

When monitoring the console output, the stdout is being redirected to do
so. If Ctrl^C is hit during this mode, the stdout is not back to the
console, the user does not see anything they type (no echo).

Add "end_monitor" to the SIGINT interrupt handler to give back the console
on Ctrl^C.

Cc: stable@vger.kernel.org
Fixes: 9f2cdcbb ("ktest: Give console process a dedicated tty")
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>