selftests/exec: Add non-regular to TEST_GEN_PROGS

non-regular file needs to be compiled and then copied to the output
directory. Remove it from TEST_PROGS and add it to TEST_GEN_PROGS. This
removes error thrown by rsync when non-regular object isn't found:

rsync: [sender] link_stat "/linux/tools/testing/selftests/exec/non-regular" failed: No such file or directory (2)
rsync error: some files/attrs were not transferred (see previous errors) (code 23) at main.c(1333) [sender=3.2.3]

Fixes: 0f71241a ("selftests/exec: add file type errno tests")
Reported-by: "kernelci.org bot" <bot@kernelci.org>
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>