sparc: mm/extable.c needs module.h for within_module_init()

sparc: mm/extable.c needs module.h for within_module_init()

And also for search_exception_tables, which arguably could be
living somewhere more appropriate than module.h

arch/sparc/mm/extable.c: In function 'trim_init_extable':
arch/sparc/mm/extable.c:74: error: dereferencing pointer to incomplete type
arch/sparc/mm/extable.c:75: error: dereferencing pointer to incomplete type
arch/sparc/mm/extable.c:77: error: implicit declaration of function 'within_module_init'
arch/sparc/mm/extable.c:77: error: dereferencing pointer to incomplete type
arch/sparc/mm/extable.c:78: error: dereferencing pointer to incomplete type
arch/sparc/mm/extable.c:80: error: dereferencing pointer to incomplete type
arch/sparc/mm/extable.c: In function 'search_extables_range':
arch/sparc/mm/extable.c:93: error: implicit declaration of function 'search_exception_tables'

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>