metag: migrate exception table users off module.h and onto extable.h

This file was only including module.h for exception table related
functions.  We've now separated that content out into its own file
"extable.h" so now move over to that and avoid all the extra header
content in module.h that we don't really need to compile this file.

Cc: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>