scripts/selinux: fix build

We need to add the object tree include directory to the include path
for building mdp in order to pick up generated/autoconf.h. Otherwise,
make O=/path/to/objtree breaks.

Fixes: e37c1877

 ("scripts/selinux: modernize mdp")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>