crush: include mapper.h in mapper.c

Include linux/crush/mapper.h in crush/mapper.c to get the prototypes of
crush_find_rule and crush_do_rule which are defined there. This fixes
the following GCC warnings when building with 'W=1':

  net/ceph/crush/mapper.c:40:5: warning: no previous prototype for ‘crush_find_rule’ [-Wmissing-prototypes]
  net/ceph/crush/mapper.c:793:5: warning: no previous prototype for ‘crush_do_rule’ [-Wmissing-prototypes]

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
[idryomov@gmail.com: corresponding !__KERNEL__ include]
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>