make_device_config.sh: Fix target path in generated dependency file

make_device_config.sh: Fix target path in generated dependency file

config-devices.mak.d is included from Makefile.target, i.e. from inside
the *-softmmu/ directory. It included the directory path, so never
applied to the actual ./config-devices.mak. Symptoms were spurious
build failures due to missing dependency on default-configs/pci.mak.

Fix this by using `basename` to strip the directory path.

Reported-by: Gerhard Wiesinger <lists@wiesinger.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>