selftests/exec: add generated files to .gitignore

Add generated files non-regular and null-argv to .gitignore file.

Signed-off-by: Muhammad Usama Anjum <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/exec/.gitignore

···
 execveat.path.ephemeral
 execveat.ephemeral
 execveat.denatured
+non-regular
+null-argv
 /load_address_*
 /recursion-depth
 xxxxxxxx*