xfs: fix missing header includes

Some of the xfs source files are missing header includes, so add them
back.  Sparse complains about non-static functions that don't have a
forward declaration anywhere.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/xfs/libxfs/xfs_ag_resv.c

@@ -19,6 +19,8 @@
 #include "xfs_btree.h"
 #include "xfs_refcount_btree.h"
 #include "xfs_ialloc_btree.h"
+#include "xfs_sb.h"
+#include "xfs_ag_resv.h"
 
 /*
  * Per-AG Block Reservations