SELinux: add .gitignore files for dynamic classes

The SELinux dynamic class work in c6d3aaa4
creates a number of dynamic header files and scripts.  Add .gitignore files
so git doesn't complain about these.

Signed-off-by: Eric Paris <eparis@redhat.com>
Acked-by: Stephen D. Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>