lib/extable.c: add missing prototypes

When building with W=1, a number of warnings are issued:

  CC      lib/extable.o
lib/extable.c:63:6: warning: no previous prototype for 'sort_extable' [-Wmissing-prototypes]
   63 | void sort_extable(struct exception_table_entry *start,
      |      ^~~~~~~~~~~~
lib/extable.c:75:6: warning: no previous prototype for 'trim_init_extable' [-Wmissing-prototypes]
   75 | void trim_init_extable(struct module *m)
      |      ^~~~~~~~~~~~~~~~~
lib/extable.c:115:1: warning: no previous prototype for 'search_extable' [-Wmissing-prototypes]
  115 | search_extable(const struct exception_table_entry *base,
      | ^~~~~~~~~~~~~~

Add the missing #include for the prototypes.

Link: http://lkml.kernel.org/r/45574.1565235784@turing-police


Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>