selinux: use the kernel headers when building scripts/selinux

Commit 3322d0d6 ("selinux: keep SELinux in sync with new capability
definitions") added a check on the defined capabilities without
explicitly including the capability header file which caused problems
when building genheaders for users of clang/llvm.  Resolve this by
using the kernel headers when building genheaders, which is arguably
the right thing to do regardless, and explicitly including the
kernel's capability.h header file in classmap.h.  We also update the
mdp build, even though it wasn't causing an error we really should
be using the headers from the kernel we are building.

Reported-by: Nicolas Iooss <nicolas.iooss@m4x.org>
Signed-off-by: Paul Moore <paul@paul-moore.com>