selftests/exec: do not install subdir as it is already created

Remove subdir from DEPS as it is already created at runtime. Without this,
make install fails.

Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Tyler Baker <tyler.baker@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>