kbd: Don't build tests

Add --disable-tests to EXTRA_OECONF as the tests are not usable in
ptest - they can only run in-situ - and fails to build when building
with -D_TIME_BITS=64 -D_FILE_OFFSET_BITS=64.

Signed-off-by: Ola x Nilsson <olani@axis.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>