btrfs-progs: Fix build of btrfs-calc-size

Build would fail because it couldn't find the usage function.

Signed-off-by: Hans van Kranenburg <hans@knorrie.org>
Signed-off-by: David Sterba <dsterba@suse.com>

btrfs-calc-size.c

@@ -19,6 +19,7 @@
 #include "volumes.h"
 #include "utils.h"
 #include "commands.h"
+#include "help.h"
 
 int main(int argc, char **argv)
 {