btrfs-progs: btrfs-crc: fix build error

btrfs-progs: btrfs-crc: fix build error

Remove the following build error.

   ====================================
   $ make btrfs-crc
       [CC]     btrfs-crc.o
       [LD]     btrfs-crc
   btrfs-crc.o: In function `usage':
   /home/sat/src/btrfs-progs/btrfs-crc.c:26: multiple definition of `usage'
   help.o:/home/sat/src/btrfs-progs/help.c:125: first defined here
   collect2: error: ld returned 1 exit status
   Makefile:294: recipe for target 'btrfs-crc' failed
   make: *** [btrfs-crc] Error 1
   =====================================

Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>