selinux: fix style issues in security/selinux/ss/avtab.h

selinux: fix style issues in security/selinux/ss/avtab.h

As part of on ongoing effort to perform more automated testing and
provide more tools for individual developers to validate their
patches before submitting, we are trying to make our code
"clang-format clean".  My hope is that once we have fixed all of our
style "quirks", developers will be able to run clang-format on their
patches to help avoid silly formatting problems and ensure their
changes fit in well with the rest of the SELinux kernel code.

Signed-off-by: Paul Moore <paul@paul-moore.com>