cmake: emu: qemu: return to ctrl-c

To avoid killing the QEMU process w/o also ending the 802.15.4
monitoring tool, we reassign the terminal's interrupt shortcut from ^C
to ^D. This change resets the shortcut to ^C after ending the QEMU
session.

Fixes #79131

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>